I have a fishing buddy, that I like to use sometimes. For shear simplicity, look at the humminbird wristwatch depth finder. You tie a little floating device about the size of a golf ball to your tube, put the wrist watch on and it gives you the depth and surface temperature. It is accurate on depth. I have to take the battery out of the watch between trips, otherwise it runs out. It is the only way to go on hike-in tubing trips. The fishing buddy gives a little more detail but is bigger and you need a holster to attach to tube/toon.
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edit: SmartCast by Humminbird is the wrist watch one. It looks like it might no longer be made. It still can be picked up on ebay, amazon, or other mail order places. Too bad because its simplicity make it very handy, even if it was a little quirky and ran the battery down.