(The save file can also be corrupted for other reasons, sometimes just bad luck. The same solution will work though)

I found the problem. Your seal is perfect, but the game must've corrupted the save file since you removed the core block.

All you have to do is press "Save Vehicle" for a new block, and paste the ship onto it with the select grid tool. Merge it to that block, and make a new save.

For some reason, if you destroy the block you start with, the vehicle's buoyancy can be broken.

hey imma look into it, send me a DM .when i fix it i will hit u up.