This is what we used.
UHF RF beacon
With a Yagi (directional) antenna we could pick up the tone from over 8km way. As the antenna is directional, once we had a tone, we had a direction. We would just draw a line on a map and then drive around until we got more tones. It sends out 3 fading tones (each tone has less transmission power) so the more tones you get, the closer you are. If you had the last tone then you were usually within 50m or so which could still be hard to find in thick bush but we could go further away and use the Yagi to get the direction and then draw 2 intersecting lines to get a more accurate location.
This all from a $50 RF beacon.
I know it would be hard to locate something like this below the sea but we used a yagi that someone could make it their back shed. I would thing with the right equipment you could make a dish to be far more directional and that would be even better for locating the thing. Then cut down the frequency the tones are sent and increase the signal strength accordingly.
All this from a $50 component that isn't in a million dollar planes.
I might have to start including my own in my luggage in the future.
Happy Days.