Recording in freshwater is getting more popular and people usually use a handheld recorder and hydrophone for focal recordings. How about passive acoustic monitoring? There used to be a possibility to plug a hydrophone to a sound meter. But this option is no longer available. What are people using to monitor shallow water environments such as ponds and streams?
The Hydromoth is becoming popular! From the makers of the terrestrial Audiomoth (Open Acoustic Devices). Actually, there was a group buy just announced so you can grab some now if interested. It's through GroupGets here. And more info on the OAD website here. Plus the publication describing it here. It's pretty affordable (as far as ARUs go) too - recorder + waterproof case = $135 in group buy.
OAD (and most conservation tech manufacturers if we're being honest) has had difficulties sourcing microcontrollers because of the global shortage/supply chain issues from the pandemic, so not sure what availability will look like in the future though.
For other manufacturers of hydrophones/ocean acoustic equipment, you could search the Conservation Tech Directory (within which Ocean Instruments is listed, as referenced in the other comment :) ).
You should still be able to use any of the marine soundscape equipment underwater. Ocean Instruments, while pricey, are the gold standard. Depending on what frequency you would like to look at, the French company RTsys also makes underwater recorders, though I have colleagues that have tested their noise floor and isn't good for academic purposes below 1000 Hz.
Also, not for nothing, but I will be deploying some SM4+Bat FS in the middle of the North Sea strapped to buoys. The microphones will get dunked, submerged in surface saltwater in bad weather. We ran a test with one unit already, and found that the IP68 filter on the microphone was pretty resilient. So, depending on what you need, you may be able to use the SM4. I'll report back about our full deployment when I know more.