The range of your hydrophones isn't affected by depth. The range is greater if you manually do the scans yourself. It will be 30 kilometers manually, though at extreme range the contacts are very faint so you need to rotate slower. The actual AI crew will only find contacts up to about 20 kilometers, which is more realistic to real life. For whatever reason sometimes the AI hydro crew will give 'no sound contact' when you ask for a report, yet you can do a manual scan and hear screws at a fairly close range.
I like to do my scans manually and occasionally you will hear a school of dolphins this way.

At least I here them in my modded game, not sure about stock.