You can get various types and sizes. with the typical collecting bag made from mesh nylon, so it drains quickly, and a wire frame to hold the top open or closed. Most have a lock so they stay shut.

When you're using a collecting bag, keep in mind that once it's full and heavy, you carry it in one hand so you can give it the heave-ho if necessary in an emergency. Don't attach it to yourself or your equipment. When you're not diving, you can use a large collecting bag for carrying your mask, fins and snorkel.

Besides their usefulness for diving in the dark at night, you'll find underwater lights have uses in broad daylight. A compact underwater light is useful for illuminating and restoring color at depth (remember that water absorbs color), as well as for looking into dark cracks and crevices (so you don't reach in without checking whether anyone's home).

An underwater light is both watertight and pressure-proof; you can take an ordinary flashlight.