honestly, i've put in more time than i care to admit figuring out how to add oars to a float tube. even when i found a stable, efficient, well positioned design that didn't compromise the structural integrity of the float tube (meaning i wouldnt cause excessive wear or damage), i had to deal with cost. a quality setup would probably cost more than an average float tube.
ideally, you'd also have leg rests which would reduce drag and give you leverage for rowing. oars also limit your ability to add a fish finder or rod holders since you really cant have anything get in the way of your oars.
dive scooters match up great with float tubes. i was sold once i tried them. you can get a brand new seadoo dive scooter for under $300 at scuba.com. i was able to get one with two batteries and a charger.
dave scadden has some great looking tubes and pontoons. they have oars attached. someone on the forum might still have one for sale in the classifieds.
pontoon frames wear out. i've looked into those extensively as well. freshwater wont be so harsh but the pontoons are heavy and have to be maintained.
look into inflatable kayaks. advanced elements makes a really great angling kayak. has tons of storage and an outrigger with 3 rod holders. there are a handful of inflatable kayaks for divers as well. you can drop in right through the middle of the kayak. i think those would work better for diving than a float tube.
as far as float tubes, i'm a noob myself. i've tried the caddis navigator and nevada and i own a fish cat 4. i'm very impressed with the fish cat. it moves really well. plus, when i nearly sank the first time out, the foam seats kept me somewhat afloat.
last thing to keep in mind, anything you throw a motor on, you need to register