When pointed at a target that is near or in the water, the laser designator's target can detach from the aim point and disappear below the water.

The behaviour isn't very consistent; it seems to vary a bit depending on whether the player is currently standing on an object over the water, vs. being on land.

This makes it very difficult to target boats, objects near the waterline, static ships etc. with a laser designator.

In this picture, you can see that the target is 5.4 metres underwater at the bottom of the screen, even though I'm aiming at a rock above water.