New to Arduino - Acoustic modem

[MarkT]

Certainly the towed comms buoy seems a reasonable approach, given the limited depth of dive, and
assuming this doesn't create issues.  It also allows easy recovery on failure.  Disguise it as a robot seagull?

[stevezee]

I've also been working on a similar project (untethered underwater vehicle) and have stumbled on a few acoustic modem options. This acoustic modem seems to be the cheapest (it's still about $1500 for each end - $3000 for a set):

https://bluerobotics.com/store/comm-control-power/acoustic-modems/wl-11003-1/

It's not a transparent serial link so you have to send data with a set of commands. There is a Python library but no Arduino library. There is a description of protocol with some examples:

https://waterlinked.github.io/docs/modems/modem-m64-protocol/

I hope that helps!