Be prepared for a touristy experience that is absolutely worth it. There is a lot of travel for a short visit in the grotto, but just sit back and enjoy the beautiful ride. First you take a nice ride on a jet ferry in to Capri. We went mid day and I recommend grabbing a bite in the marina. Then you take a launch to the grotto (you will see signs for the launch when you get off the jet ferry). Once you get to the Blue Grotto you have to wait your turn to get into the tiny row boat. Once in the rowboat (which can be an adventure in itself) you have to duck all the way to the bottom of the rowboat to get into the grotto. It is breathtaking, you have to see it to believe it. Tip your rowboat driver before you go in so he will serenade you and not bug you about tipping.
Take the jet boat from Positano to Capri
Take a launch service from Capri to Blue Grotto – you will see signs for Blue Grotto when you get off the jet boat
Transfer into a small row boat to go into the Blue Grotto – the same launch waits to take you back to Capri