Rocket, if its just curiosity, fair enough...
I would be very surprised if you could just lease a seaplane, much less, one of the types you have mentioned. Generally seaplanes are difficult to arrange to use, unless you own it, or are very well known by the aircraft owner. I suppose that anything can be leased, but you should expect the costs, and insurance costs, to be extremely high. Unlike an landplane, the insurer
knows that you're taking it to a place
other than an airport. Very high skill and experience is required by the pilot to deal with all kinds of variables of the route and landing site, and if something goes wrong, the plane is likely a long way from help or a road. Add to that,
if its an amphibian, there is the added risk that it is landed with the wheels in the wrong position.
Insurers are getting so picky that when I renewed the insurance on the amphibian I've had for four years (and insured with that broker - no claims or accidents), he said that it was only because I had a current policy, that they would renew. They are very reluctant to write new policies on amphibs.
To be insured on the types you mention, you would need many tens, to a hundred hours on type. I can't help you much with costs for these. I have flown them, but was not paying for it. Most of the people that I know who have these aircraft take the "money is no object" approach to operating them. I would be very surprised to hear that a IO-550 C 206 amphib went for less than $500 per hour, and I would expect to hear prices from $1000 to $1500 per hour for a Caravan amphib, but that's my guess, not based on formal information.
You can do 172 floatplane training close to where I live, $215 per hour
Lake Country Airways: floatplane training, endorsements, float charters and rentals I know there is an business in BC who trains on a 180 floatplane, no idea of the cost. I've never heard of training on anything bigger.