[AllyCat]

Snow or water - it's the same...

Not really; Snow has a white Diffusing surface whilst Water is "transparent" with just a few percent of light Reflected at the air/water interface.  The "verticality" of the laser beam could be quite important.

It might depend if the surface of the water is disturbed (e.g. whilst filling the tank) and on its transparency / depth, etc., but of course a white "float" (e.g. a constrained table-tennis ball) could work very well.  Generally, ultrasonic ("Time of Flight") methods are considered to be NOT very satisfactory for water-level detection.

Cheers,  Alan.