At first, i also thought "ok, gimmicky hack with just under water movement everywhere". But under the surface (hah), i was positively surprised by an interesting flow of upgrades, novel ways to tackle existing layout, completely new/adjusted item abilities, and some fun little references to other games and meta stuff. No tricks required except basic underwater stuff and crouch jumping, which is required very early in the game. Some sweet sequence breaking is also possible though, if you know your suitless tricks. The only thing i really found less than obvious was Grapple's location, that one eluded me for a while.