OK, let's make sure we understand what you want. You do *NOT* want an "underwater photo" but rather a photo of the surface of the water that also shows the model underneath the surface.
First, you need to decide where you want the illumination. I suspect you need to illuminate both the subject under water and the surface of the water. If you didn't want to "see" the water surface you wouldn't be going to this much trouble.
I doubt you'll have a lot of success trying to wrap up a bunch of flash units in plastic bags and tape. Check out lights made for underwater pool illumination at a pool store. Barring that, look at flash bulbs rather than xenon strobes. (Make sure you have M sync available. X sync won't work with flash bulbs. But bear with me till the end.) There was a fairly famous underwater photographer whose name escapes me now that used flash bulbs all the time. Kept them taped to his legs. So they will work underwater, as counter-intuitive as it sounds.
The next thing I expect you'll need is illumination specifically for the surface of the water, not the model under that water. You probably need to control this pretty well to get your surface effect like you want, and the only thing I can say is experiment. Obviously there must be some small motion to the surface or it will be invisible in the final image. But you don't want so much reflection that you cannot see through the surface. So I would guess that you want to illuminate the surface obliquely to create small amounts of specular reflections without overwhelming the image.
Now, depending on your format, and if you take my suggestion to use flash bulbs below the surface of the water, you might get away with a xenon strobe above the surface if you have a flash solenoid you can use to trip the shutter. Get an old style press camera flash bracket, fire the flash bulbs and the solenoid from the flash handle, then fire the above water xenon strobe from the x-sync on the camera/shutter.
Of course, all of this might be way too much trouble.
