usually when doing the tourist-type snorkeling, you will be floating on the surface while wearing a life jacket. the bag would would just fine. however, you need to remember to use a lanyard to secure it to yourself, or you will find that it will sink quite rapidly if you lose grip of it...
if you are more confident in the water and am a strong swimmer, you can opt to go without the life jacket - which allows you to dive down for 30 seconds to 1 minute depending on how well you can hold your breath. How deep you go depends on yourself. I have free dived down to 10m before but I don't spend more than 10 seconds at that depth. Again, the bag would do just fine. Except that you may find that the pressure is depressing all your camera buttons at the same time, effectively locking it and you cannot operate the camera properly. that's why the instructions say that you should put some air in it to counteract this phenomenon. However, too much air and it will be too buoyant at the surface, and you cannot bring it down deep.
Anyway, as long as you follow the instructions and make sure it is watertight, it will work for your snorkeling trip. Good enough lah.
Can lend me after your trip? hehehe