I've seen ads for handheld depthfinders that resemble a small radar gun. Not sure how well they work and I don't remember where they are sold, but that may be one option. I've used one of the Fishin' Buddy depthfinders and they work pretty well but you'd probably have to rig some sort of mount, and the plastic tube with the transducer may be a problem in a float tube. It also stopped working for some unknown reason after a few years of use, and I didn't want to pay the $75 plus shipping to get it repaired.
Here's one to avoid: someone gave me a HawkEye Portable Fish Finder that runs on AA batteries and has the transducer on a cable with a float. Unfortunately, the part with the display, controls and battery compartment is not waterproof
. The electronics and display are kind of lame too: it has fake bottom contours drawn on the screen so the picture on the box looks like the device is plotting the bottom contours, but in reality it has little "fish" images that supposedly show the depth and weeds so you have no way of knowing what the fishfinder is "seeing".