I beat the game today (Adult mode). There are techniques you can use on every boss, the hard part is finding one =) Once you identify the patterns most bosses can be taken pretty easily.
SEMI SPOILER (if you don't want to know anything)
One important factor for any boss is the Slow+Zoom+Punch, its the most damaging thing you can do. You simply Zoom in and hold the punch button. For the Shark boss you'll want to do as much damage as possible before he raises the water level. After he does raise it, there is some trigger (which I haven't fully discovered yet) that will make him summon a bunch of water mines, if you get him to chomp on one of those (and then punch him) the water level will go back down.