As Jeremy notes, there are two issues with height: Torso length, which governs headroom issues, and leg length, which has an effect on panel clearance. I've met guys who were really tall and had long torsos, and could not fit in an airplane that another similarly tall guy could easily fit in, because that guy had long legs.....not a long torso.
I've flown an M-7, an M-6 and an M-5 all on floats a fair bit. As Jeremy says, kept to legal gross weight, they are all great performers, especially the ones with the "universal wing". But, even the M-5 performs well on floats, assuming it's got the 235 hp engine (which is the only one I've flown).
The tradeoff, however, is load. Find one on floats, and perform a realistic load calculation. Our M-7 on Baumann floats with full fuel (as in full mains and full tips) was essentially a two place and some baggage airplane. If you could keep the fuel load to mains only, it was a good three place airplane. But, the mains aren't all that big, so range is limited.
If you're even considering salt water ops, I wouldn't, frankly. I'm not convinced that the corrosion proofing on any of the Maules is up to that.....which of course is true of many production rag and tube airplanes.
Finally, there is simply no comparison between the Cessna 180 and the Maule, on floats or on wheels. And, I'm not suggesting that one is better than the other. The Maule will kick ass on any 180 performance wise, but the 180 carries more and has a much better range capability, and there are other ways they are very different airplanes. Both GREAT airplanes in their own right, but....
Not meaning to bad mouth the airplanes at all, they can be a great two place or minimal three place airplane on floats, and performance is great.
And, Jeremy, whoever told you that a Cub won't come out of Alexander Lake loaded (and I'm pretty sure I know who that is) is full of it.....I've come out of there at 2000 pounds a number of times with Cubs, and maybe a time or two a skoshi bit over that.
Nevertheless, the Maule is a great performing seaplane.
MTV