If Invaders or Dahan are moved into Oceans they instantly drown. What about Beasts, Disease, and Wilds? Obviously they won't be much use there, but they could have some value for powers like Strangling Firevine, which deals damage per Wilds in adjacent lands.
Also, what about the presence of other Spirits? Normally they can't place presence in the Oceans because Oceans aren't treated as Coastal Wetlands during Growth. But there are several powers that place Presence on the island, and those could target an Ocean. Some of them, like Indomitable Claim, are about as useful as a Disease token in the Ocean, but Flow Like Water, Reach Like Air becomes fantastic if you can use it to drag towns and cities into the Ocean.