My biggest reason to think we might actually end up with polder islands floating out in the sea is because the graphics support it, so it won't break anything. Using the magic of firetuner:
The right middle polder is a legal polder. The Bottom polder was placed in the middle of the sea. The left polder is placed on flat land, and the top polder is placed on a hill tile! The lump of dirt the windmill sits on acts like land does- it will grab towards other land, or if you make a whole nest of polders in the ocean, each other:
Its not perfect but its not like illegal polders have goofy graphics. I'm more impressed that the flat and hills polders properly contort the terrain!
Obviously the devs could change the rules but iirc even in firetuner changing tiles is myopic - only twist is you have to reload the map or run a script to render in-game terrain changes from water to land or vice versa. They will have a cleaner way of doing rendering when tiles submerge for GS, I am sure. But right now improvements only get deleted on city capture if they are unique or nuked. There's isn't anything that evaluates improvement deletes based on the condition that they could still be built on that tile afaik; so they would need to code in something new. That's why i compared it to mines by seaside resorts- under the hood it would be the same.
It's another thing that can go wrong and would need time to fix. This is the company that adamantly would not fix code typos like the infamous 'yeild' table after release until they fixed other bugs. The Strategic Air Force policy card is still missing the "%" symbol from its text, for example.
Argument via path of least resistance. I personally would love it if they really tinkered with each civ for the new expansion to make sure things are as fun and connected as they can be with the new mechanics, since they put so much effort into making unique civs in the first place.