pommiesteve, like Bill says, yes you can book your own boat, and not necessarily a taxi boat. we've done it a couple of times and it makes a nice change - we also paid £10 each and the kids were free
bear in mind though that you need to check the boat out first as some don't even look seaworthy - also the cooking hygiene is a major consideration
again, I'd suggest
ICR (like always ha ha) would be able to set you on the right track or maybe even arrange it for you?