Diving

It takes effort to swim on the world map. When you overexert yourself, or your movement points run out, you'll be unable to keep yourself afloat and sink. You can dive under water (go down while swimming). When you do this, an extra display is added to your prompt, showing the amount of air you have left. When this runs out, you will start drowning. To avoid drowning, you can cast aqua-breath on yourself. Pets with the ability to breathe under water bestow you with the ability to breath it as well, as long as you are mounted on them.