Originally Posted by BogMonster
They shoot pretty much straight down I think,
Beam angle of a generic transducer is (as I recall) on the order of 20 degrees or so. The unit shows the first returned echo from within that cone as the "bottom". It doesn't know (or care) if it's pointing down or forward or whatever. If it's pointing severely forward or backward, you get a deeper-than-actual depth reading, unless you don't get a signal back, in which case you lose lock.
The Echopilot thing looks like a side-scan unit turned 90 degrees. Neat device.
Knowing how deep the water is behind you is always something I have regarded as of limited use when going into somewhere you are completely unfamiliar with
True, but even if you knew what was under the bow (i.e. a forward mounted conventional fishfinder), you have to be going slow enough to a) see the display, b) react to it, and c) actually get the boat to avoid it.