This feature is only available in Custom Islands, and is no longer in-game. The Sea Fort is a structure in Stranded Deep. It is bigger than most Shipwrecks and contains much more loot than one as well. It is often quite dangerous to get there and explore, as there is often many sharks patrolling the area. Additionally, the player can get Broken Bones from falling high heights in the structure.
Some good items to carry with you would be:
2 Splints
2 Bandages
6 pieces of any food
A decent sized Raft