Glad I found this thread before starting my drive. I do underwater photography.
The bags do work but are not really rated for anything past 30 feet and if you snag them on a rock or ledge then your camera is toast. What kind of camera are we talking about? I have a D800 and found a dedicated Ikelite housing for 500 bucks used, with the port and everything. I re-sealed it which was cheap and you can do if you're not stupid.
You're not going to like this answer but outside of snapshitting, pay up for the hard housing for your camera, especially if it's something nice. I used to cheap out on this and used underwater compacts for most of my work and when I went to an Ikelite housing and put my D800 in it even with older AF-D lenses the difference was night and day.
The other thing to consider about the bags, while they do work, if this is a salt water environment, the salt gets into every single opening and you have to clean the bag very well after each use. They work but for anything above basic snorkeling, get the Ikelite. I'm so glad I did. You will use it again and when you step up to something in my level you will actively be looking for places to take it once you get hooked.
As for your lens, you will want to go wider than 50 as the water acts like a teleconverter. I use a 24mm lens on 36x24 and it works out to about 28mm. A good underwater housing will correct this with an UWA port made for wide lenses. The 50 will be nice for shallow pools if you're shooting fish and coral beds and stuff, but you'll want something longer for those unless you have a +30mp sensor and at that point you'll want the dedicated housing anyway.
Another bonus to the housing is they make the camera neutrally buoyant in the water.
Some advice for underwater, check your equipment the day before your dive. Try to keep everything at the same temperature as the water. If there is a great temperature differential you will fog up. Test and check everything the day before.
