I bought one of those little pressure operated depth gauges years ago and it works just fine. Reason I didn't reply earlier is that I have not seen them advertised in tackle catalogues or websites for some time.
For boat fishing, all you need to do is load 40 or so yards of backing line on to an old or cheap fly reel, and tie the gauge to the end. In practice, pull off 15-20 yards of line and just lob the gauge in the oggin (keep hold of the reel

). You'll know when you've hit the lake bed when the line no longer goes straight down, but some lies loose on the surface. Reel up, take your reading and empty the gauge.
As PA says, you can find out some surprising and worthwhile information about what lies under the surface.