I know we do (Protech), I'm pretty sure Ecologic do (all their gear was new in 2006). Not sure about the others, but a lot of rental gear on the island looks ancient. You can also snorkel with people like Searious and Seduced, who are tour but not dive operators, and they may also rent gear for private use. Reckon on somewhere around US$6/day. Remember though that a mask needs to be agood fit for your particular face not to leak, and of course if you don't have perfect vision you may benefit from corrected lenses in the mask. You won't get lenses to your prescription anywhere other than having them made, but you may find a reasonable compromise off the shelf - eg. we have a few of typical prescriptions.
Actually, I would always recommend you buy at least your own mask, and ideally snorkel and fins as well. Don't forget something on your back, as people get very sunburned whilst snorkelling. A lycra rashguard is ideal, but a tee-shirt works pretty well. And don't forget a wide brimmed hat.
You may find it most convenient to rent from near your hotel. Where will that be?
AmbergrisCaye.com 
