Mild Badtides (suggestion)
Right now badtides are always 100% badwater, which makes the best way to deal with it often making it go off the map as quickly as possible.
Which if you employ that strategy, makes it basically a drought, except shorter, since badtides are balanced around being harder than droughts, and so all difficulties have them as shorter than droughts.
So i think, in custom difficulties, you should be able to change the badwater percentage for badtides, affecting water sources and water seeps.
So you could say set badtides to be, for example, really mild at 20% badwater (80% water) but make it much longer than droughts, which would challenge you to keep the contaminated water.
To combat it, Maybe you'd have to put your crops closer to the river so they don't dry out. Have a drain for your dam so your water pumping doesn't concentrate the contaminated water into too much badwater. Make your roads that go in the water have gates to turn them off to prevent contaminated beavers. And, late in the game, have mechanical water pumps to separate the water from badwater
Also please let us save custom difficulties