Having a finder on a float tube is pretty important in my opinion, unless you simply plan on fishing a water you know well or perhaps something like a canal where the only real features are probably visible without a sonar. In lakes, reservoirs and large river impoundments I think a finder is near essential. For years I used one of those old portable units, that was neither very portable or particularly good- but it did enough of a job that I could find underwater structure and know the depths of where I was fishing. The main issue was that it was still pretty heavy (over 2 kg with the battery) and the mount, for my tube at least, was pretty cumbersome. Although heavier PVC tube that a mount could be permanently fixed to would probably make a big difference on that front, but then you'd have a heavier tube and that's not a road I really want to go down. The whole benefit of the float tube is that it's light, stealthy and can be launched anywhere, some guys set ups are so beefy that I can't help thinking they might as well have a boat instead.
The problems with the old finder meant that I would frequently choose to leave it at home if I knew roughly the depths of the places I was going. Even the best sonar will struggle to pick up ledges and drop offs when it's left in the cupboard, never mind an ageing portable unit. The Deeper fixes all of these issues though. It's light, the phone mount, transducer and old phone weigh in at less than 200 grams. It's easy to attach to the tube, having no mount it can just be attached with a leash to one of the D rings and left to float around. I made my leash out of 2feet of 60lb braid, so it probabl ads a gram weight. This is a huge advantage on a small inflatable that is leg powered; 1 it's less to push overall, and 2 (perhaps more importantly) it doesn't affect the balance of the tube in any way. There's no external battery for the Deeper unit and a charge is supposed to last around 6 months-but I'll need to investigate that. All the Deeper units are screenless and you need to use a phone or tablet to show the graph, this is a bit of a double edged sword in my opinion. It provides a nice colour image and you can split the screen to show the graph and bathymetric map-the app has a basic offline map that it then improves as you cover water, this can then be saved to the cloud to be improved for future trips. Deeper also has a subscription service, and you get 3 months free when you buy a unit but I've not registered for that yet so can't relly comment on it. There are some problems with this screen system though. Firstly, smart devices aren't designed for water, proper finder units are. I can easily see phones getting killed while being used with the finder. Then, there's battery life, I'm using an old phone that I've factory reset and taken the sim card out of and so far the best I've managed is about 4 hours of use, OK I can carry a small power pack and it's still lighter than the old finder, but it's adding something else to carry and another thing that can potentially fail on you. The biggest issue is the screen function, touch screen is great unless you've got wet hands or water droplets get on the screen, I've found that sometimes the session will stop or some setting will change without me wanting it to. Also although the screen is colour, smartphones aren't designed to be viewed in direct light. This means that it can be hard to read the screen depending on conditions, again not a problem with a true finder screen. This last part can probably be fixed by adding some kind of shade to the phone mount, but I've not got round to that yet.
Overall I'm pretty pleased with the Deeper. Although I would love it if they made a small lightweight screen with a good battery life to go with the transducer unit, I think the advantages in weight and the ability to add mapping to the floattube system outweigh the drawbacks. It's not perfect, but so few things are.