depth-finder for ski boat on shallow lake

You can pick up a cheap depth finder - like a Hawkeye - for around $100. Just know that these cheaper depth finder only find the depth pretty much directly under the transducer.

And have realistic expectation of what they can an cannot do. If the lake goes from 6' to 1' within 60 feet and you are traveling 35MPH you are covering that distance in only about 1 second. That can get you in trouble fast.

Check out the Navionics app.

You can use your phone to see your location over a map of the lake which is marked for depth.

This app can help you find the deep water and stay in it at speed.

I fish a River with a narrow channel surrounded by shallow water. This app keeps me floating.

On my ski boat- Direct Drive- I just have a depth finder with a 2" gauge. But that boat spends 99% of its life on 2 stretches of river where I know the depth.

If I was exploring lakes I'd buy a fish finder with maps.

I like lowrance. Plenty of them used for cheap if that's a way you'll go. If not just buy a 5" or 7" graph that has maps installed or accepts a map card and install it. They're so cheap now that everybody is going to live view.