Have my 3rd party windsurf insurance with Viking but on my last sailing/windsurfing holiday their holiday insurance wouldn't cover us for 3rd party liability for dinghies or yachts. Noble marine can tailor insurance to cover pretty much anything you like.
By studying the small print and some follow up phone calls I have found some standard holiday insurances which will provide the same cover at a much lower premium but I have no idea how claims would be handled.