[Tyhm]

I'm unclear on the concept...is the former Zormite Castle entirely underwater, or partially underwater...?
It might be kinda cool if you made the first swimming jump-and-run. Just apply inertia to the player's x and y movement; it's supereasy, just figure out their speed and add to it all the time, I'll give you the basic script for it:
in a (invisible/system) NPCW's timeout loop:
if(!playerx=this.oldx){
this.playerspeed+=(playerx-this.oldx)/5; //slow start
while(abs(this.playerspeed)>2) this.playerspeed*=0.9; //top speed
this.playerspeed *=0.9; //for eventual deceleration
this.oldx=playerx;
}

Then just adapt the gravity script for falling down slower....
Then you can have like parts of the castle underwater, and/or parts you have to swim to get to, vaguely reminiscent of Zelda: Link's Awakening.