If you want 3rd party only, http://www.basic-boat.com
may be worth a look. Otherwise, fully-comp "off-the-shelf" policies like Craftinsure look like good value, as long as you fit the criteria (basically under 50mph) - I guess your boat does. For other recommendations do a search as this has been discussed many times before.
Meme, why shouldn't someone consider 3rd party only? If the loss of the vessel is acceptable/affordable then 3rd party is the way to go... but then for a few extra £'s you can cover several £k's worth of craft.