The operation of small floatplanes or flying boats from salt water has two main downsides (authority and traffic aside).
The corrosion is a big issue. Yes, with vigilant washing it can be overcome, but this washing presupposes that you have a means to get the salt wet plane out of the water, and heavily flushed with fresh water right away. Taking off from the sea, flying to an airport and applying a garden hose stream a half hour later is not going to do it. Those who are successfully operating from salt water have a lot of infrastructure in plance to make it work.
Secondly, and more to the point is wave height. Sure the sea can be calm sometimes, but not often. For any seagoing aircraft of 4000 pounds or less, wave heights exceeding 12 inches total are the practical limt. You can do more with skill, but it just pounds the plane, and should be reserved for circumstances where you have no choice. Even if the sea conditions are decent, the wakes of vessels super impose. I have flown a Teal (very similar to a Lake amphib) off a virtually calm lake, to encounter three super imposed boat wakes which just about ended the career of the plane. No damage, but a testimonial to the fine low speed characteristics of the Teal, just as the Lake. I have had similar scares in other types, so I am very weary of rough water. I have flown a Cessna Caravan amphib on and off 12 inch waves, and it's much better, but still hard on the plane.
I was once the backseat observer to a spontanious landing in Lake Ontario, in waves I would estimate to be 2 feet or so. The touchdown was well flown, but the water so rough, the plane just slammed repeatedly, and he went 'round. When he finally landed in the calm water, and attempted to takeoff, the plane would not accelerate at all - it was sinking. We taxied up a (thankfully very close) ramp, to find that the floats were so badly bent and wrinkled that many of the rivets had been sheared aft of the step, and they had the seaworthiness of a seive. I excused myself from this unhappy scene, with an unforgettable lesson on what not to do.
In Canada, the many small lakes offer many areas in the lee, and bodies of water with small fetch, so you can find your spot, if you've been trained how. It is uncommon to see floatplanes on the larger lakes at all, unless they are in bays and other secluded areas.
The references to operations in Florida are generally operations from a number of smaller fresh water lakes there. There are some saltwater places where you can get in and out in a secluded bay (in the Keys), but certainly not along the main coastal shores.
As romantic as float flying is, it really is only practical (on the small plane scale) in smaller lakes, which are not too busy, or noise sensative.
The other thing to remember is that if you're landing a floatplane, you are very nearly never at an airport, or even airplane freindly place, and could be somewhere totally on your own. The slightest maintenance problem gets big fast. Can't get it started at the airport can be managed. Can't get started on a secluded lake 50, miles from a road, is going to be a major and costly inconvenience!