The video I watched about floating cities did mention something about rising sea levels. That works well with part of my premise.

They also used some clips from Water World in the video. It was kinda fun.

Also, after I watched that video, I said to myself "Oh yeah! Chinampas!" Aztecs built the original floating cities on a lake near where present day Mexico City is. They were made of matted vegetation and the Aztecs built houses and farms on them. They're cool!

I still like the idea of having a massive network of interconnected ships though. I'll figure it out even if I have to draw a map.