If you are snorkeling on the surface, and the bottom is more than 4 or 5 feet under you, it is unlikely your strobe will be able to light up your subject enough to help the colors the way you would desire.
If you can hold your breath, get a foot or 2 away from your subject, compose the shot, then fire, then blast for the surface...then the strobe(s) will make a big difference, especially for 20 to 30 foot deep subjects.....the 6 feet to 12 feet zone can often be "color balanced" in Lightroom sufficiently for the strobes to be unnecessary.
2 strobes is much better--one for the back ground, one for the subject and foreground....Shooting with one strobe generally makes the object appear as though it is lit up, but the background is too dark--almost like a night dive.
Freedive fins, some freedive tricks( learn from a good freediver in a few hours in one day) to make 20 to 30 foot drops easy and would make photography at 20 feet easy enough....
Strobes are not cheap, but pretty much all photographers that want good underwater shots, use them.
For Scuba diving, the likelihood is that you WILL get significantly deeper than 20 or 30 feet, so the strobe will make a very large difference--the colors with the strobe will be much less washed out--far more saturated and "popping" with the strobes...more like what you saw and want to remember
