Years ago, I fished from an inertube float. Today a new breed of personal fishing 'boat' has become etremely popular, Kyaks. The biggest drawback that I find when using these forms of water travel, is limited space with no means of mounting electronics in what space there is. Trust me when I say I've seen about every idea concieveable for doing so. Some involved adding to the existing 'boat' while others incoorperated towing some sort of floating platform behind or connected to the side. Those many years ago, I began using one of the cheapest and easiest depth finders that was small enough to carry in a prescription medicine bottle. Not only was the space problem taken out of the picture, so was the guess work of determining depth.
This miracle little jewel of a tool is called a crankbait and when used has the bonus of actually putting fish in your 'boat'. Think about this: If you fish from a single person vessel you likely only carry one rod and reel combo. You aren't out on the water respooling that reel, you do that at home and likely use the same line each time. What you have is a constant base to use when determining not only depth, but cover and bottom composition. Just add a crankbait. You don't even need to know how deep the one you tie on runs although it helps. Add a shallow, med. and deep running crank and you can find everything from drop offs to bottom composition changes, wood, weeds....... you get the idea.
If you're really looking to use electronics and not experiencing the simplicity of fishing from these forms of enjoyment, go out and get one of those goofy remote units with the bobber thingy and the Dick Tracy wrist viewer or whatever, but a word of advice if you opt to go that route........ your smart phone will interfere with the signal when you're texting mommy what time you'll be home and asking what's for din din.