Swim
Walk in until you start swimming. Push the stick the way you are looking, so look up to rise and down to dive. Hold A to rise, and move toward a low bank to climb out. You can also pull yourself along with sweeping strokes.
CarefulYou can only hold your breath for a short while. You get a warning as it runs low, and at the end you start taking damage. Reefmaws live in deeper water.
Ice
In deep cold, still water freezes over and you can walk on it.