All well and good. But im kinda making a ssp map/pack so server plugins wont work. It kinda all hinges on something that can force players to stay under a certain y level until later in the game. Unless I can raise the height of the ocean to 256 and then lower it later on this seemed like the easiest solution.
Spaman did something like that with his 'Marooned' map - only in that map it was keeping people above a certain level until they passed a certain point. There were layers of unbreakable blocks, with certain structures that poked through those layers.
(By the way, I have a passion for building ridiculous underwater bases, so if you need a tester...)