Make sure the waterzone box covers the entire playable area.
I usually make it the same size as the ocean
Also the ocean closed spline, make it as simple as possible, but I think it is a good idea to hug the inside of the coastline.
Ideally lakes and rivers should be in the ocean spline hole so the waterzone doesn’t mix up ocean water behaviour with lake water behaviour…which it did in the past but may be fixed now.
I like to put the last river spline points on an ocean spline point
Another thing to try is perhaps the actual player bounds may do something, have you got Blocking Volumes around the island to contain the players.
I haven’t used Map Controllers yet so I’m unsure about them, but disappointed to learn they have a 500 tile limit. They are a newer device so probably still getting tweaked a bit,
Its a pity they don’t pick up where the minimap fails