The water will be fine. As mentioned above, it may be chilly for a local, but the beaches have been loaded lately w/ people, who have all been swimming just fine. If I went out into the Ocean on boat to snorkel, I would need a wet suit though. You have a few options though that are not out on a boat that you may be fine w/o a wet suit. On singer island, there is a public beach called Ocean Reef Park. Behind this park is a nice snorkeling spot when it is low tide.
Another great option would be to snorkel on Peanut island. You can catch a water taxi from Singer island for 5 dollars each way, and hand out on Peanut Island. They have a great snorkeling trek there that is shallow, and you will have the opportunity to see lots of cool stuff.
Have a nice visit. March is a great time of year to be here.