[dylan604]

but why do buoys or sensors need to transmit that data underwater? They could easily do that via radio transmissions above the surface.

to me, anytime underwater communication is concerned, it is solely to be in the realm of military submersed vessel communication. There's no compelling reason for any other communication to undergo the expense of underwater transmission.

[azalemeth]

I guess the other use case that comes to my mind is the oil and gas industry who have a lot of underwater equipment, which could conceivably use an acoustic modem as a backup device.