Thanks MickeyM.
Would the ‘ping’ have to be sent vertical, I.e. straight down? If yes, that would eliminate ultrasonic for me as the sensor would have to be on shore so as to be in a constant fixed position. Or do I have the concept wrong? Would the ping get returned by the water surface or the lake bottom? Perhaps a little robot rowboat once a day to a constant spot?
I’ve run across some sensors bases on pressure, which would work great but are outside the budget. A couple of hundred I can sneak by. Much more than that would probably get noticed and may effect my quality of life around the house 
So much more stuff to learn
I think I found the answer to my (now dumb sounding) questions thanks to following NorCal Dan’s link. Looks like ultrasonic would have to sit above the water via an arm of sorts to measure distance to surface. Looks like I’m stuck with something based of pressure.