My guess is that we will get one new character exclusive to this game. Also I was hoping to be one of the girls instead and then hanging out with the others but yeah... Anyway I hope they still add volleyball as a feature
I'm doubtful they're going to make an exclusive character for DOAXVVP, not unless they shut down DOAXVV in the near future, and we haven't gotten any hint at them having plans to shut down DOAXVV yet.
So far as Prism, as long as they don't rehash the same mistake they made with Xtreme 3 by overpricing their DLC and thus singlehandedly killing any chance at more girls getting in (the PlayStation magazine hinted in the interview that they would add more girls and activities to Xtreme 3 down the line), I'll go along with 6-8 available at launch (even though I personally prefer they just release all of them).
The graphics almost remind me of the Resident Evil Engine, though, so not sure what to make of that.
I won't be surprised if they do add jetskis into Prism, though: Sakuta DID indicate that part of the reason he did that Dolphin Wave collab was to test the waters to see if a jetski minigame could be added to the game.