Ok for shallow water stuff but still cannot replace a strobe(s) because the natual light coming from the sun is coming from (in general) one direction, which is from the surface of the water. With a single light source from the sun, a dark shadow is casted on the opposite side or below the subject, no way around this unless you use a strobe to light up the dark areas or have someone hold a reflector sheet on the opposite side of the subject to bounce the light into the dark shadow areas.
For clear shallow water, wide angle stuff like ship wrecks, where a strobe cannot cover the entire image this may help out.
Also as stated, this filter will decrease the light input and as the saying goes "dull lighting yields dull photographs"