I found this report from 1996 analyzing the results of tests conducted by SAIC to be quite interesting from a couple of points of view. The first is to see what the state of the art in airborne LiDAR technology was in the mid-1990s and the second for me is the fact that it sounded like it was going to be commercially feasible for large fishing fleets to use helicopters equipped with LiDAR to locate schools of tuna and other valuable fish. For some reason, at least as far as I know that is not being done, but I guess I could be wrong on that.
The report indicates that the LiDAR was theoretically capable of penetrating the water column to depths of 450 feet in blue water. The researchers felt that on average a depth of 300 feet was needed to be commercially viable.
Now if we could get this into a handheld version the hobby fisherman would be all over this.