Hi! So, sadly we haven't gotten much if any setpieces for the ocean. So i've compiled a whole bunch of ideas for set pieces! I really hope we could get some in the next update, whether they're my ideas or something else.
Broken/Ruined Boats!
These are a simple but effective idea! Set pieces containing broken and ruined boats with different look and resources on them. I think this would be a wonderful addition to the game.
Sea Weed And Sunken Chest Set piece
This set piece would consist of a circle of Sea Weed sorrounding a sunken chest! You can pick up the chest with a pinch n winch but once you do all the sea weeds will agro and start to attack the you and the boat!
(Note: The Chest could/should have better loot than the chests you can find from bottles, to make it that little bit more rewarding)
Random Waterfalls
Just some random waterfalls as a neutral set piece! It doesn't do anything. It's just for looks.
Seasonal Fish Shoal
This shoal is just like any other shoal that's able to spawn Malbatross like usual. But with a small twist of instead spawning seasonal fish depending on the season! Fallfounders in Autumnn, Ice Breams during Winter and so on.
Rockjaws and Strident Trident
This one is pretty much just a rip-off of the classic hound and staff set piece But I think it'd work well!
Cookie Cutter And Boat
This set piece consists of a singular boat with many boat fragments spawned around and a bunch of cookie cutters around. Nothing too interesting other than the cookie cutters attacking the boat and eating the fragments. But it adds to the oceans environment!
Edit: And thanks to a friend for helping me come up with some ideas, 
And that's all i've got so far. I'd love to hear any ideas anyone else has!