The little Humminbird units, either straight depth/fishfinder or comboplotter, are effective and economical. I just have a Norcross Hawkeye depth finder on my Whaler, simple little < 100 unit, company has great service. We take it pretty far afield, as much as 20 miles from the mothership, but I get by with the relevant area of the local chart in a sheet protector, if that. We love to explore by dinghy, and we have always viewed the big boat as delivery vehicle for the Whaler or other dinghy (kind of like the space shuttle on the back of a 747), it has never occurred to me to install a plotter. It would be nice in some places like the mangrove mazes in 10,000 islands in SW FL, but that's the only place on either coast I can think of off hand where it would be really useful. I have used the dinghy a few times to survey a potential passage or anchorage for the big boat (again, SW FL being a good example), so that's one application where a sounder beats a lead line or oar-in-the-water method.