That's really a great idea for a guide imo.
In one game I was built around a lake.
I needed half a year to get from my cap to a province two provinces away and the water nation (Agema) could raid me in 11 different provinces in just 1 turn.
Ironically I got a Barrel of Air from a lucky event so I just traded everything for getting me underwater, + used indies and attacked him sneakily.
He kicked me out of the water in less than 4 turns I think.
I guess I could have done better but even then it would have ruined me.
Some general points:
Since you need water breathing on every mage you can never field as much as him.
Many attack spells don't work at all and your enemy is unlikely to research evocation. This could mean that it's easier to build thugs, if you get something amphibian.
Mostly you get two types of underwater recruitables - chaff and heavy encumbrance - exploit that. Otoh many of the indie provinces allow you to recruit a lot of units quickly (low resource costs) and they are not much weaker than the recruitables of most uw nations.
Mobility - movement is limited to 1 province.
Just raid him and avoid his armies - he has a hard time catching you.