BFT administrator and moderator
The PiranhaMax is a popular sonar model among tubers, tooners and yakkers. It provides all you really need to know...until and unless you become a sonar geek and have to have more bells and whistles.
I have been tubing over 50 years and have been using different models of fish finders for at least 30. Like anything electronic, you get what you pay for...but you can pay for more than you really need. I like the dual beam transducer to allow efficient readings in both deeper and shallower water. I gotta have good grayline...to help determine the bottom composition and structure.
Efficient display of fish is important too...to know their relative size, depth and numbers. Helps to know how deep to fish and to be able to guess what species they might be. Some low-end units show bubbles, algae and other inanimate objects as fish targets. What appears to be a large school of fish coming up off the bottom often turns out to be a sunken tree.
We don't need speed indication on float tubes but having a temp readout is very helpful. I keep a log of all my trips and part of each trip log is a notation as to the beginning and ending water temps. And on some trips you can find a cove with water temps a few degrees higher than the main lake and that is where the fish will be. Or just the reverse. During hot summer conditions you might find the fish around a cool incoming stream...or an underwater spring.
It usually takes a few trips of watching the screen and correlating what you see with what you find by bouncing bottom or catching (or not catching) the fish you see. After a few trips you get to know your favorite spots on your favorite lakes much better.