Hi all.
I live in PEI, and can definitely tell you...there is no warm water here. Ever. Middle of July? Still way too frikken cold! I think the warmest I've ever recorded, on the hottest day of the warmest summer since I started diving over ten years ago, was in the range of 20 degrees...and yes, I know that's not bad, but remember, that's the BEST it's EVER been, and that was near the surface. Drop down twenty feed, and you hit a wonderful thermocline...
Despite the cold, I love diving here, but there are some other downsides. Visibility is only fair at best, and if you're used to Caribbean diving, you'll be disappointed in the lack of bright colors. Shore dives are also problematic, simply because our shoreline tends to slope very gently, making it difficult to find places where you can get beyond 15 or 20 feet of depth from shore without a very long swim.
That said, by all means plan to do some diving while you are here. Diver's Quarters and Black Dolphin Diving are two shops based in Charlottetown that could hook you up with other divers who would be happy to have someone to go for a dip with. I work for Diver's Quarters during the summer, doing commercial diving with aquaculture. I know I'm almost always up for a trip, and there are plenty of others who are generally willing to go out for a couple of evening or weekend dives. Once we have a group of six or eight, we can generally get one of the fishermen to take us out, and compared to most places, you'll probably find a charter quite inexpensive. There wouldn't necessarily be a dive master, though, we'd be on our own to explore.
And there are some nice spots to explore. I know of a couple of wrecks that make nice dives, especially the Tunstal, located in about 75' of water a short sail out of Covehead. She's not intact, but there is plenty of debris, and a few large sections still recognizable. There are some nice areas off of New London and Stanley Bridge (near Cavendish) as well, with plenty of marine life to see. (But remember...no bright colors!)
Hope to hear from you...or anyone else who is on PEI and looking for a dive buddy!