What happens if 1 or more Invaders/Dahan moved into the Ocean aren't successfully destroyed by the Ocean in Play special rule?

What happens if 1 or more Invaders/Dahan moved into the Ocean aren't successfully destroyed by the Ocean in Play special rule?

Push them to non-Ocean lands (as part of the same action). If the Push also fails somehow (perhaps due to Isolate), the pieces will be left in an illegal state and cleaned up to the nearest land after all triggered actions complete – human pieces cannot remain in the Ocean between "action trees" (a primary action and all other actions following from it). Regardless, Ocean's Hungry Grasp does not get Energy for them.
This isn't formally specified on Ocean's panel because it's virtually never relevant with just the core game, and spelling it out would both take up a lot of space and needlessly increase complexity / confusion-potential for players who only have the base game. However, it becomes very relevant when playing vs. The Habsburg Monarchy Lvl 4+ (Jagged Earth), which blunts "Destroy Town" effects; see this FAQ if you need details.
The fact that the Push is part of the triggered action means that it can trigger further actions – e.g., Melt Earth Into Quicksand or Call to Guard in the land Pushed to – which cleaning up illegally placed pieces does not do.
As a very specific edge-case, if the Ocean is also a normal land due to Weave Together the Fabric of Place, humans can hang out there without trouble (until end of turn when the effect expires).