Once you're in the water, hold both mouse buttons together and move the mouse, you'll find yourself pointing in that direction and heading down, or up, depending on whether you've rolled the mouse forward or backwards from the centre point. Practice a few times in shallow water before diving deep, just so you don't run out of air and get stuck!
If there is a fissure in the bottom of the lake bubbling, you can swim around that and get 'fresh' air from that to recharge your breath - otherwise you need to go to the surface and get air there.
Sometimes when mining and something decides to attack you, you can easily forget about air and run out whilst trying to get back to air - in these cases, I usually swim to the surface and let the thing get a few bites at me, and attack it once I get air in me. It will often follow, allowing you to hit it - or leave you alone, which is even better.
I've noticed that there's usually a couple of mining points in the bottom of each of the pools in the Barrens.
aj