I've done it may times. My technique is not simple and requires some decent breathing control!
You need: thin rope, swimming/diving goggles with or without nose part, snorkel, food grade extender tube for snorkel.
First thing: tie a thin rope to everything you can: prop, c-clamp, spanner.
Extend your snorkel with the tube to desired length, attach it to your boat over the waterline.
Spend loads of time underwater learning to breath IN WITH YOUR MOUTH - OUT THROUGH YOUR NOSE.......why? If you extend your snorkel and breathe normally the CO2 builds up IN the tube, because your lung volume does not fully expel all the used air while exhaling!!!!!!! this is important, if you don't do this, this technique is potentially lethal!!!! as you could become senseless and drown.
Once you've mastered this, it's also a good way to clean your hull.
Have fun - BE SAFE - have friend there with you in the water!!
Remember that inboards (not all i think, but most) have this little piece of metal between shaft and prop that will fall out!! do it in shallow water.
This technique does not allow you to dive much deeper than 16-18 inches due to the water pressure