A different world event idea here. The Siren Penta-vault.
The penta-vault is a non-naval world event, and creates a situation where multiple crews are trying to do a mad-dash to collect as much loot as they can from various vaults, while trying to steal loot from the other players.
Boats sail to the penta-vault world event, and interact with a mermaid statue. The mermaid statue brings the ship under the sea to a secure dock, and a mermaid statue that will take individual pirates to the penta-vault itself. Each ship gets their own statue, (there are 6 to use and they crumble once used), that leads to it's own separate dock. Inside is a vault trial, similar to what's in the siren vault. Though, these waves of enemies primarily serve as a delay tactic allowing others to join the event. The final vault boss drops a key item that can be placed on a pedastal in the center of the main chamber. This starts opening up the vaults themselves.
The vaults are spread out from the central chamber through a series of inter-connected tunnels, and possibly light platforming (similar to some of the siren shrines). The treasure must be brought back to a mermaid statue (similar to the ones in shrines) that match the color of your dock/decending statue, there it is placed with a mermaid at your dock that you can claim your treasure from. All roads between the vaults and the mermaids will eventually take you through the main chamber again.
The vaults themselves open in waves. The first and second waves open two separate smaller vaults. The third wave opens up a larger vault. Inside the main vault is the fabled [insert treasure name here], worth a valuable amount of gold.
Once you have emptied the vaults, (or earlier if you want to make for an earlier escape). You can follow your mermaid statue back to your boat, load your treasure, and then talk to a different mermaid to have your boat released to the surface...let's hope nobody is waiting for you.
