So I'm running a one shot and have decided the dungeon (temple) will be underwater, long forgotten. I want getting into the dungeon to be a puzzle and have resonated with the following concept but want to sanity check peoples thoughts on it being fun and not too easy.

A gargantuan beast I've not in my mind specified is perched on the temple entrance

Core Points to this

• The first encounter will show it biting and eating a killer whale in one single and rapid bite.
• There are currents which will grant difficult terrain/free dash (depending on direction) going deeper towards the temple.
• The temple is 90ft underwater
• Not sold on this concept. But I was thinking like every X seconds you hear/feel vibrating your bones a deep resonant growl. On a DC investigation check, the party see other sea creatures near the creature halt movement as this happens
• Not sold on this either. Maybe the best is stirred by light? DC perception check? Bioluminescent creatures gets its attention? I don't know yet.

Effectively the puzzle in my mind is how to slip by/bypass/avoid or in some capacity get by the gargantuan sea creature without combat.

My main goal is to present a puzzle that is fun. I've thought of many challenges due to water and none seem fun to solve or work around. Like the lack of light, immense depth, oxygen challenges or anything else. So I'm looking for ideas on how to maximise this being a fun challenge to beat, not a drag and not a cakewalk

It'll be 6 level 5 players.

I've never done travel or similar in DND so decided for my campaign to utilise it a lot. I'm running a method which is working pretty well but I feel doesn't fully capture what can be done with travel and I'm wanting some ideas or pointers to help go from a solid foundation to a player impressing session

My current system

• Travel is classed between close to very far (credit point hat) where there's a number of encounters for the distance category. For example, 10 and 18 miles could both be close and thus have 1 encounter.
• The encounter either progresses the story, gives more information on the world or is specific to the region/environment.
• I don't track rations, water or resources similar.
• I don't really do watches during long rest unless its specific to a planned encounter.
• Though technically I made some random encounters, I never use them. They're always planned in advance (The situation that is. How the players deal with it is up to them).
• I run a dilute version of gritty realism where they need 3 long rests in a singular location to get the benefits of a proper long rest (Dubbed True rest for us). This solved all the balancing issues with this style of travel system where there could be 30 days of travel dealt with in 3 encounters.

Where I feel I want change

• I'd like resources to matter more. Maybe not when doing known travel routes but for unknown wilderness or something. But I don't know how to implement this without it getting bogged down in a boring numbers game.
• I'd like to give more Feeling to travel. I'm effectively hand waving so much environment and although I don't wish to be bogged down in a hex crawl of descriptions, I'm certainly lacking here somewhere.
• I want skills to matter more. Currently I'm failing to ask travel speed (I'm so bad at remembering this exists) and never ask them to do any checks during travel.

Effectively I think what I'm missing in my heart is what video games give (Like Skyrim) where they can see the world and see things, Points of interest if you will, and have the capability to explore it if they wish. The problem is, when I've described one particular thing they can see, they (reasonably and understandably) think its quite important and want to go interact with it. So I suppose what I'm wondering is what systems, methods and ideologies have worked for others?

Side note. I'm aware a hex crawl fills in a lot of this, but I'm outvoted everyone to me in that case. I suppose what I seek is like a Hex-Crawl-Lite where some benefits from it are applied but in a system where the session itself isn't travelling. A good middle ground.