Well, the trick is to zap them first, before they get you. Extend your hand with your alternate air source in it out in front of you, sneak up under a nettle, then give it a blast of air. They go shooting up to the surface and explode. When the nettles get really thick, this is how I have to clear a path in order to move forward. It sort of reminds me of doing mine sweeps in Viet Nam.
And the best part is you'll entertain all the paddle boarders up top.