In between Positano and Praiano there is a little tunnel that if you find, you can swim or paddle right through to the other side..You could just about get through it in a small boat. I filmed it going through with my kayak once:
There is another cave just to the left of the Africana nightclub, it's easy to find, the entrance is almost perfectly rectangular. You can drive in with a small boat, just be careful in case there are swimmers inside. Once in look back at the entrance, you get that magical light effect similar to the Blue Grotto on Capri.
Aperitivi, so go sit down and have a drink!