Some games with mechanics like the one you're implementing do this. https://www.gamespot.com/articles/e3-06-rise-and-fall-civilizations-at-war-hands-on-single-player-hero-mode-naval-combat-and-the-ancient-world/1100-6147762/    

The only reason they have a range bonus, is when I tested it, it seemed like you had to get really close for the units to actually attack I didn't think about swimming. Maybe if they had like drowning effect, and can't fight, but you can send them to shore, or garrison them on a boat

Are you able to provide ressources? This would underly your advocacy attempt and may lead to faster implementation.

Yes, this change makes this simplified passage a bit more accurate. But instead of 'fell', 'lost its independency' would fit more.

They should swim or die, depending on where they are.