Move is a preexisting function :P Try renaming it.
Also, you can just do something like this:
PHP Code:
function MoveFish() {
if (!onwater(this.x+1.5,this.y+2)) return;
this.return = int(random(0,50));
if(this.return > 45) {
// stuff
}
}
'return' will stop the function immediately.
Also, it may be a problem with some sort of level thing... I doubt it, but try something like:
this.level.onwater(x,y);