Found this great little simulator yesterday, Virtual Sailor
. Mostly a sailing simulator, but there are powerboats available (there's a Zodiac RIB around somewhere on the internet, though it's not that great). There's a demo version available for download, but that limits you to about 5 minutes playing at a time. Full version costs $25 and seems worth it.
As well as extra boats for download, you can get new scenery packs. Downloaded the England pack earlier and had a quick cruise through the Menai Straits, great fun. There's also multiplayer functionality, so if the weather's a bit too crap outside, have a virtual cruise together!
Also, I started work on a decent RIB for it earlier, so hopefully in a week or two that'll be available!