I think it depends on your 'stomach' for taking risks with sensitive camera equipment. Most of these 'packs' are rated for splashing, such as on a boat. Submersion is a different issue, as it is not only water but also pressure. This is the reason for the jump from $140 to $1400. Of course, you won't be diving to 200 ft when snorkeling, but you very well may be doing 20-30ft, if you want shots of coral and fish near the bottom. While the Aquapack above says it is rated for 15ft, I have had a IPX8 rated GPS get water damage in the floor of my kayak. While Garmin replaced my GPS, I doubt Nikon or Aquapak will replace your 5100.

In addition, what happens if you drop your camera and it sinks to the seafloor? This may be 30-50 or more feet.

Finally, recall that it is much darker underwater, and decent shots usually are going to require flashes, typically external flashes. Kodak used to make (maybe they still do) a film disposable camera that was waterproof to 10ft. However, you find when you get your photos back that a flash was absolutely needed.

My suggestion is to consider one of the Fujifilm, Olympus or Pentax rugged waterproof cameras. These go to 30 ft, are impact and sand resistant, have built in flash and have underwater modes to correct for color. And alot cheaper than $1400.