I just bought annual cover from Journeywise - www.journeywise.co.uk - they not only cover for scuba diving (within normal bounds of the sport), but were also by far the cheapest of the companies I had quotes from, whilst providing a comparable amount of cover. Obviously I haven't had cause to claim from them yet - they might have appalling customer service, but fingers crossed you'll never need to find out! Enjoy your trip . . .