Handling offshore sailing for your party
So your party decides to do some offshore sailing. and don't want to book a passage on a fully crewed ship Here's how I handle it without needing homebrew extensions or making it boring.
For a boat used by only your party, go with a bigger fishing vessel type or the Bristol pilot channel cutters or Norwegian fishing vessels. Those boats around 35 to 45 ft length are very capable of crossing oceans with a small crew. Example: https://seilnorge.no/boat/goxsheim/ or https://woodenships.co.uk/sailing-yacht/bristol-channel-pilot-cutter-5/
Usually the boat will average 4 to 5 knots (nautical miles per hour) for a total of 100 to 120 nautical miles in a day. On really good win give them 150 nm a day. If you want to give your party panic, becalm them for a day or 3. This is when they go with the wind or across. Against the wind they need to zig-zag which reduces the way made toward the destination to half. Those are modern numbers, but should work well for DnD.
Magic: Being able to make / change wind is really, really powerful. Magic forecasting the weather makes a passage a lot safer. Being able to create drinkable water and food gets rid of one of the major problems of longer passages. Magically knowing where one's destination lies is on long offshore passages a big boon too. If one can fly, he might see a lot further. Mending is a spell I wish I had on my boat.
Things to do on a boat:
Keeping the sea-water out of the boat: Not an issue in light and good weather. In heavy weather, 2 people working one hour of emptying the bilge every 12 hours. CON and STR check to make it in half the time. In a storm (bad weather) 2 people need to get rid of water all the time (or half the time if they make their CON or STR check). Check every hour.
Helming and Sail handling: It's something to keep them busy without needing much checking. Helming gets a boonus from DEX or WIS, Sail handling from STR or DEX. Check once or twice a day for how well the crew does (the best roll counts for all of them) and penalise them a little by speed (10% for less speed than optimal if they don't do that well on sail handling, a little off-course for helming) Helming needs skill checks on difficult passages close to rock or entering shallow bays. Sail-handling only of reefing in heavy weather or storms.
Navigation: I'd make it and INT or WIS taks, whatever the player prefers. Before the invention of reliable watches, you could know how much north or south you were by the position of the sun or the stars with a sextant (or similar) but you didn't know how far east / west you are. If they need to find and island, you can get indications by clouds, birds, and changing wave direction, changing water colour. That's how the polynesian wayfinders (look that up on youtube) did it. If some can speak with whales or dolphins, they might get some low to medium quality directions too. Check once or twice a day to see how well they know where they are.
Weather forecasting: It's and INT or WIS task, done at sunrise and sunset. They'll know know what to expect for the following 12 hours and can prepare the boat in case of heavy weather. If they move towards a calm area or are becalmed, they'll have an idea which direction is the quickest exit. If they're good at forecasting when approaching a calm area, I'd allow them to constantly move at 1 or 1.5 knots while failing the navigation check they'll move into the becalmed area without movement. For heavy weather / storms, weather forecasting will allow to avoid the worst and reduce breakage. Weather forecasting and navigation might yield different courses in calm spots and heavy weather / storm. Forecasting tells how to get out of the calm, even if it isn't the perfect direction.
Worrying about drinking water and food. Calculate with 3l or 1 gallon of drining water per day and head on board. If water is lost or gets bad, that's really bad. WIS check to keep up on rations and catch problems early. Also keeping pests under control like rats and weevils keeps the food and water unspoiled for longer. Purification and all those useless Druid / Ranger spells might help. Fireball too, but it might have undesired side effects. Every week let the healer keep scurvy, sun exposure and other illnesses at bay.
Sea-sickness: I'd make that a fun one. With the first heavy weather or storm, everyone rolls CON for sea-sickness. Those with a sea-going background can skip it. Give characters with a plausible coastal background advantage. Those pesky dwarves, elves and tieflings get disadvantage if they aren't the sea-going kind. They will feel like dying and are basically useless for tasks on board until they make their skill-check. Check twice a day. Sea sickness can't be healed, but a healer will give them advantage at their roll and make them puke less. Once they come out of it, no more sea-sickness for this voyage until they go on land for more than 3 days. You might require a seasickness check for characters who really don't like the sea on the first day instead of the first bad weather.
Fishing: supplies food and is cheap positive thing to do. A wholesome activity for any time except heavy weather or storms. When becalmed, all sizes of fishes can be caught, when under way at 4 knots or more, only big ones. Or whatever you feel is fun.
Resting: Short resting, between watches and outside of heavy weather and storms, but no long rest for the party. They need to take watches around the clock, I'd require 2 on watch and more or less all hands on decks for big sail changes (eg reefing and preparing when heavy weather comes). As the boat moves constantly. If they want to long-rest, they need to be at anchor in a protected bay. You might also give them one long rest if they're becalmed for more than one day and they make their CON check.
Weather: Make a forecast check (WIS or INT) every sunrise and sunset. If they feel bad weather coming, they can prepare and nothing bad will happen. If they get caught with too much sail, there's a little breakage or something goes over board. Or the boat simply takes on sea-water which needs an hour by 2 people to get rid of (STR and CON). If you're nasty. let the sea-water contaminate some of the fresh water. When a bad weather is about to hit them unprepared, let the helmsman at that time do a perception check. If he fails, you might even let the main navigator to do a perception / weather foere-casting check with disadvantage. He might sense it in his bones. That might give them the time to reef quickly (with a sailing-handling check) to avoid the worst. No need to be an arse about it, let the party have some successes.
The more the party prepares, the less the damage, less water. It's doesn't have to be catastrophic. Reducing sail for bad weather when nothing is coming, penalise them with 25% less speed. The more skilled they are, the closer to go at optimal speed without breakage.
Becalming is a difficult weather on an open ocean passage. No progress while feeling hot (or cold) and using up food and water. They can fish, they can repair stuff, the can wash and dry their clothes (makes them more comfortable and give them a small boon). They also can easily get the excess sea-water out of the bilge. If you're feeling really generous, they can do a long rest on a successful CON check.
Heavy weather / storm: Don't overdo this one. If they get caught without preparation, some sail or spar damage to start with and 2 hours of water into the bilge. They will need constantly to get rid of sea-water (2 people working in a running washing machine) while the bad weather lasts. 2 people to helm the whole time. Maintaining the course is really, really challenging, running with the wind will alleviate the effects, but they won't know where thy are after the storm. Some sail / spar breakage. After the storm, they'll need 2 days of work in in nice weather to repair the boat. Don't sink the boat except in storms with really bad luck. They might lose a mast or shred sails and they'll have to rig something temporary (2 to 3 knots of speed only for the rest of the voyage).
Spare parts and repairs: Assume the boat has some spare wood, spars, ropes and sailcloth to repair the damage of one storm or 2 unprepared heavy weather breakages. Skill in carpentry, sewing or similar will make the repair better. Tools should be available on the boat too. Don't be an arse about it, but not everything can be fixed.
Combat:
Being attacked by someone: Assume the other captain knows how to approach them and their own navigation skill is amateurish as they don't have a skilled captain. It will be boarding and close combat as any encounter. Fireballs are very devastating on wooden boats, so the enemy will have some way to throw them. Very deadly on the open sea, close to shore better know how to swim and not have any metal armour.
Attacking some other ship: If they can't trick the other ship to let them come close, they forget about it. Their lack the skills for naval combat will allow the other boat will just outmanoeuvre them or lob a fireball into their sails.
Boarding another ship in a bay at anchor: That can work, if they plan it well and is fun.
I hope this give you some practical ideas for your campaign