Sorry for jumping in so late.
Four things spring to mind:
1) The navy, as mentioned by cassimer. It's likely to be a while before you're unleashed on the hardware, but it is quite good hardware!
2) The RNLI, as mentioned by cassimer. The only problem here is that they are all volunteers except for the mechanic, who is more likely to be in the bilges than driving. Wrong end of the throttle cables I suspect. Also, it is going to be a long time before you can actually drive their stuff, but it is REALLY good stuff, better than the navy's!
3) You aren't going to like this: fisherman! Ask at you're local fishing port but you'd probably need to be self employed to get to drive. It is also one of the worlds most dangerous jobs.
4) Rescue boat driver at a sailing school. This may not actually be a bad idea! You'd be pottering more than tearing around and it might be fairly seasonal. Look at people like
http://www.uksa.org/
5) They just keep coming! Driving the pilot boat at your local major port. You'd get to drive a mile or two offshore where you drive into a ship (in a controlled fashion of course) to collect or deliver a local pilot. I have no idea how you'd get into that but your local Vessel Traffic Services (VTS) people can help and are usually friendly.
6) Sometimes the local police can have a marine or waterway unit that have their own boat. That would be a good job, but you'd need to go in as a normal copper and apply for the post if/when it arose and I'm sure there'd be LOADS of competition for that job!
Whatever job you go for, you're going to do a course or two. Have a look at
http://tinyurl.com/bsbf7