If you only need
If you only need depth, then I can highly recommend the little Hawkeye depth finder. If you want a fish finder, the Hummingbird Piranah is a reasonably inexpensive unit. I have no experience with the Hummingbird, but they sell a lot of them. Don't think it has a keel offset though. The Hawkeye is just a 2" digital readout. It does have depth alarm and I think an offset for the keel. Be careful about the keel offset, and if the unit you end up with doesn't have that feature, make a lead line and check the difference at the dock, so you know that what ever it's reading, you need to add x feet to the total depth, or subtract for the depth under the keel. If you get one with a keel offset, then just set it, using the same method, and you will get a direct reading of water under the keel. I like to have mine reading actual water depth, as it is no problem to figure the keel into the equation.