Single MOST important tip for rolling...
LEARN YOUR OFFSIDE ROLL! Any roll will do for starters. Sweep (aka "screw"), C-to-C, whatever. But before you spend time learning a whole bunch of rolls on only one side, master the ability to roll on _both_ sides. It's likely that you'll find yourself against a rock, or in a hole, where the current will prevent you from using your "preferred" side. By not having a "preferred" side, you'll swim less often. You'll get heckled less, too - it can be embarrassing to swim from relatively calm water next to a rock because the rock + current wouldn't let you use your "preferred" side.
Practice your offside roll until you can't remember which is your "onside" and "offside".