I use an older Humminbird Fishin Buddy 120 on my float tube and on my frameless pontoon. It works great for identifying water depth and bottom structure and I always use one for stillwater fishing. The angle of the bottom scan beam is a little narrow for identifying fish, but the side scan can be useful sometimes. Unfortunately, they are no longer being sold. I use strap-on Humminbird Fishin Buddy mount on both the float tube and the pontoon boat.
The Garmin Striker 4 seems to be the most popular choice of the currently available models. Lots more features than you'll probably need. A Scotty 268 strap-on mount can be used for display and the transducer and the battery can go in one of the float tube side storage bags. Fish Finder Mounts website has some fancier (and more expensive) options for battery mounts.
https://www.fishfindermounts.com/float-tube-mounts
Ken