Desire has sparkling water from a mexican bottled water company. It's perfectly good, I like it! I have never seen any San Pellegrino, but that doesn't mean there isn't any somewhere. If you really need maybe just stop at Costco/Walmart or whatever.
Agree with SJ, its perfectly fine, we have had it at Pearl and RM (Suki & Sahlo)! Another option would be to ask one of the butlers, they can get you just about anything for a price. Lastly, a quick run to the market in PM (Puerto Morelos) and you'll find what you need.....20 min walk along the beach or very short cab ride.