As is the case with any flying instruction, it is inescapable that the major cost is the operation of the aircraft. Thereafter, the quality of the instruction may be related to what you're paying the instructor. Very experienced instructors don't often give their time away. And, add to that, seaplanes are unusually expensive to insure. This is because the insurer knows that nearly every place that aircraft lands and takes off will not be in the "controlled" environment of a runway at an airport. Worse, if there is a problem just severe enough to render the aircraft not ferryable, it's going to cost lots to get the plane home. Whereas, that problem in a wheel plane was much more likely at an airport, near a road.
For those pilots I have seaplane trained, the seven hour minimum was not enough - to meet my standards. The guidance I work to for training requires a minimum of 25 hours of dual. My recommendation for their hull insurance will not be accepted with less. That said, those 25 hours did not afford the opportunity for the new pilot to broadly experience "away" operations. Bush operators know this, and I suppose, keep a keen eye on their newer pilots, that's up to them.
The only shortcuts to the cost of obtaining the entry level skills in water flying are to do lots of reading about the techniques, so you're not spending time in an expensive plane, understanding something which can be presented on paper, (and maybe the occasional video clip). Next, learn the basics in the lowest cost floatplane you can. The bigger planes can come later. If you can handle a Cub, Champ, or old 172 well as a floatplane, the 180+ will come fairly easily later. The skills are fairly portable between types*, and the skills to get a low power floatplane up and out will be good for you when you're later flying a loaded 180+. * Flying boats are very different, and a different set of skills. In my opinion, they should be a different rating than a floatplane. If you can, build a relationship with a floatplane owner. Personal mentoring goes a long way for new water pilots. That was my success - aside from the closely supervised 5 solo circuits (which were not insured, he just took the risk on his private 180 for me), I did not fly solo in a floatplane for well over 100 hours - all dual, and learning. Thereafter, I was "lent" the 180, when doing so suited the owner's needs.
Consider buying a very modest floatplane (Aeronca Chief, etc.), Forgoing hull insurance, and doing your training and experience building in it. I did that, buying a low cost amphibian, when I needed to accumulate a few hundred hours of amphibian time to be easily insured on a very much more expensive amphib.
Thank you PilotDAR for all your insight and suggestions... I agree that 7 hours seems way too little to have a good handle of floats operations... Many years ago (90's), I read the Marin Faure's book "Flying a Floatplane" (which I plan to re-read before I start the float rating, of course) and it gave me an almost intimidating respect for floatplane operations, so I have no illusions that i will be a "piece of cake". I like your suggestion of getting started on low cost / smaller floatplanes, not so much for the low cost, but for what you said "the skills to get a low power floatplane up and out will be good for you when you're later flying a loaded 180+". Good point!