Finding dolphins and whales

I've covered quite a few kilometres looking for whales and dolphins, and I have decided that I am doing it wrong. The sea is very, very big, and from my deck I can scan only a very limited area. Even worse, the animals that I am trying to find spend much of their time submerged - I should be looking under the water, not over.

I've heard whales and dolphins on hydrophones before, and it was possible to get a sense of range simply from the volume. You can hear them a long way off, too - loud animals, and sound carries very well through water. What I think was missing from the experience was stereo - with stereo, it should be possible to estimate the direction from which the sounds are coming - and so steer closer.

What I would like to do is to trail two hydrophones astern of Briongloid and hook them up to a pair of stereo headphones. Now, sound travels much more quickly through water than through air, which might make it hard for the brain to process the data fast enough - but a simple fix for that would be to increase the spacing between the hydrophones so that the time sound takes to get from one to the other in water matches the time sound takes to get between human ears in air.

Since Briongloid is usually powered by sail (rather than a noisy engine), I imagine it would be possible to trail my pair of hydrophones throughout a voyage, monitoring constantly for cetacean activity. Interesting idea: now how do I actually go about building such a thing?