I use them quite a bit and have for years. Our water doesn't get much above 10 degrees C (50 F) in the summer. I didn't know Victoria water temperatures would get low enough to need a vest.
Generally, I wind up buying several out of catalogs (favorite is Cabelas) before I get one that fits fairly well. Sometimes you have to trim the arm holes as they can rub the fur off a dog if they rub against him when he walks or runs. You'll have to trim around the male parts on some of them too. I just bought a 5mm thick one size L from Cabelas that fits my 10 month old 22 inch high (~56 cm) 55 lb. (25 kg.) lab bitch. I'm going to buy the XL for my 23 inch (~58 cm) 67 lb. (30.5 kg.) lab male.
If you buy from somewhere else those sizes won't mean much as they vary one heck of a lot. They all probably come from China anyway.
I don't like the 3 mm ones to much as the dogs tear them up pretty fast. As if the thicker ones lasted much longer.