[hackerspaces] Experience with sonar technology, anyone?

[riot]

Hi hardware hackers..

I am wondering, has anyone here ever played with sonar technology or even built
something useable? I'm interested in fish finders as well as good technology to
map sea/riverbeds.. even only single components.

Searching sadly didn't yield too much information, apart from commercial (very
expensive) stuff.

Cheers,
riot