You’ve probably hear the island referred to as St. Johns, with an “s”… heck, you might even call it that. One thing I am sure of is that the island is definitely St. John, singular. And I don’t know much about the two St. Johns, so I thought I’d take a quick look.
The first is St. John’s, on the island of Newfoundland, on the Easternmost edge of Canada. Here’s how it looks:
This is St. John's, Newfoundland (Canada)... not exactly a Caribbean island paradise.
Though St. John’s is right on the water, their beaches are “pretty rocky and cold” (according to one web site), and the average high temperature in March is a brisk 34°F (1°C)… so this is not the place you want to be in the winter. 
Then I found Saint John, New Brunswick (also in Canada). Here’s a look at Saint John:
Saint John, New Brunswick - want to swim in that water?
Saint John is also on the water (in this case, not the Caribbean Sea, but on one Bay of Fundy), and in addition to being the first incorporated city in all of Canada, Saint John also said to be the most Irish city in the country, as well. Saint John also shares St. John’s very chilly winters… and I’m not sure that I’d recommend it as the place to go for great snorkeling. 
Here’s a look at Saint John, St. John’s and St. John on a Google map.
And finally… just to finish with a “palate cleanser” and to get those other Saints out of your head… here’s a nice look at Turtle Bay on St. John, in the lovely U.S. Virgin Islands:
the beach at Turtle Bay (photo: Fred Hsu)
And incidentally, the island is having fabulous weather… today, March 23, we had a high of 80°F (27°C). Now that’s more like it!
So the next time your friends ask you when you’re heading back to St. John’s, politely correct them and let them know there’s no “s” where you like to be. No “s,” and no snow, either.