S100 and S&S yes-01 work in underwater mode

I am interested in adding a strobe and am wondering if the ys-01 will work in dummy point & shoot underwater mode with my canon S100?

Hmm. That's the setup that I use but I normally shoot in manual or aperture priority. If the camera fires the flash in this mode (in some modes you can't shoot with a flash) then the strobes will fire as well.

But the camera won't know that there will be more light from the strobes, it will try to adjust the exposure to the on-board flash, so I think that your pictures may be washed out. Maybe it will work for some background fill but not for the foreground.

Shooting in manual mode sounds difficult and imposing, but you can set the camera to something like f/7, 1/125th second and then adjust the position of the strobe and its output to get a good shot.

The camera does flash in that mode if I set it to. So, maybe ok if I'm not shooting macro?

Forgive my ignorance-- I'm a beginner who does find manual almost frightening.

Oh, don't worry! No apology needed. We are here because we want to learn more.

You can give it a try. My wild guess is that the pictures may be overexposed because the camera is not aware of the extra light coming from the strobe.

Some cameras are smart enough to do that, but in those the strobe is connected via a data cable to the "hot shoe" of the camera (where the flash attaches), so the camera communicates with the strobe and tells it how to fire.

With these strobes the cable that attaches to the housing acts only as a trigger. It lets the strobe know that the camera flashed and then the strobe fires.

Again, definitely, get a strobe and give the underwater mode a try, but then switch to manual mode, use the rear spin wheel to set the camera to f/7 and the front spin wheel to set the speed to 1/125" (don't worry too much at this point about what this means and what effect they have). Aim the strobe so that there's some side light to what you want to shoot. Shoot a picture and if it's too dark increase the power of the strobe. If it's too washed out decrease the power of the strobe. And feel awesome because you're shooting in manual!

If later yore curious about what the different positions of the strobes do or what the F stop or speed mean let me know and I can send you some links.

And one good thing to do is to set the programmable button on the back of the camera to manual white balance, but that's something for another day.

Low resolution example of a shot taken with the Canon and YS-01: