See Malthousemarine.com, mention YBWF for a 5% discount and ask for a Garmin 140 fishfinder. He's a) on this forum and b) VERY helpfull. No connection, just bought from hin and very happy customer!
I agree; no point for a specific depth meter if you want to do some fishing occasionally. Get a fishfinder which will give you depth as well, and also easy to interpret for soundings.
It's worth getting the best you can afford, as reliability and accuracy is a more or less direct function of price, and a good set of data from soundings can be a very useful navigational aid, almost like underwater radar.
i'd agree ,a good fishfinder and the transducer will work set up as an inboard,stick the transducer in a bag of water move it about til you get a good reading,get a suitable bit of plastic pipe fix it to the hull with fibreglass or epoxy,make sure its sealed,put some oil in it ,pop the transducer in and away you go,
been using one for years.