For what it's worth I took out some insurance with Newton Crum (picked at random from an internet search results list...) I have no idea if they are any good but the policy was pretty comprehensve and covers stuff like theft from your house, secure storage at your clubhouse and from a vehicle so long as it is locked inside or secured with a locking device to the roof. This alone makes it worth looking into as household contents insurance rarely covers waterborne craft such as boats (which your lovely new board would be classified as..) Accidental damage is also covered including launching/ landing (ie trashing your kit in the shorebreak at Langdale ) or watching your board launch itself off your roofrack on the M3 on the way there..
The policy also includes £3M third party liability.
None of which I hope to use but for £44 to insure £1400 total value of kit I thought it wasn't to bad ?
Alternatively I think membership of the RYA is roughly the same and includes £1M third party liability for free if you nomnate windsurfing as one of you interests when joining?