Regular digital cameras under water?

Experiment with something else of about the same mass and weight in the bag first to see if it truly is water tight and flexible enough to operate necessary controls. The bigger enemy to digital cameras is not the water so much as the sand. Don't get sand in it, that kills many cameras every year!!! If the bag is right up against the lens, it is near invisible and more or less just acts as a neutral density filter, much like the so called "sky filters" that are really on there to protect the lens. Your point of focus will be well away from there so even the auto focus and exposure should ignore it.

You can get digital camera that are rated to a few metres ( ie 5m) under water and take reasonable photos.

To go any deeper you need specialised equipment like a camera housing and strobe. These are expensive.

Zip lock or vacuum sealed bags won't work. They are not waterproof and any salt water that gets into your camera will wreck it.

Remember that the pressures involved quickly build. For every 10m you add 1 atm of pressure

try olympus.......750....