Do not be misled by the assertion that "it won't work underwater." That appears to be a long-held belief, seemingly plausible, but apparently unsupported by experiments.
As noted in this thread, it turns out that, in addition to the project linked in my post 17 (above), others have done underwater tests of a relatively inexpensive and easy-to-use ultrasonic sensor intended for use in air (such as I suggested in my post 17, and such as encouraged by post 18).
It works and appears to work well, for measuring underwater distances up to 50 feet (claimed by one experimenter), with just a little bit of "fiddling" to make sure it is waterproof (especially the connections to the sensor)...
...unless they are not telling the truth about their results. That's possible, but it seems unlikely.
Note: this does not mean that achieving your goal of "communication" between two otherwise unconnected underwater sensors will be easy. You will need to have a deep understanding of the TX and RX electronics for the sensor you are using. As deep as that for the HC-SR04 illustrated in the two links mentioned in my post 25.
And, depending on your underwater environment, your sensors could be confused by ultrasonic noise from other sources (dolphins...)
Edited to remove the blue-green laser remark and to include the link to the subsequent thread on the same topic, which has links to three other users claiming successful underwater use of ultrasonic sensors intended for air.