A large offshore oil rig could create a new PvP hotspot at sea. With high-value loot, multiple levels, and interior areas, it could offer varied combat and finally create more boat fights on the open sea.
However, players returning to the coast could easily be camped in the open water. Adding small islands, rocks, or other natural cover would provide alternative routes and safer ways to return.
As a simpler alternative, a massive container ship could also work.
As an additional feature, there could be a new boat or a craftable raft. The raft would be slower but much quieter and could require manual paddling.
Seagulls would also be a nice detail. They could appear over the sea or around fishing villages, indirectly revealing interesting locations and making the ocean feel more alive.