My wife and I took an afternoon trip with these guys - no booking necessary, we just turned up and paid. We both had a great time, it's a marvelous way to see Waikiki and Diamond Head from a different angle. The boys offered up bits of information here and there and were always available to take photos for you.
You can have the drier ride sitting in the catamaran or ride up front on the netting where you're guaranteed to get soaked. There's also a stop out at sea just before the end of the trip where you can have a swim.
Oh! And take your own drinks.