The minimum required runway length for a Cessna Caravan is *762m/2500ft. Considering that the Caravan has a stall speed of 60 knots, and for a safety margin, you would approach at 70 knots, this makes sense. Assuming that it slows at a rate of around 1 knot/ 10m, You can do some basic calculations: 70/1, which is 70, resulting in a 700m long runway, plus 62m for a safety margin.
EDIT: But now we're talking about amphibious, so water could help the plane slow down, but at the same time a longer takeoff roll would be required. And it turned out that the website I used wasn't reliable. Sorry about that. Anyways, it is said that an amphibious Cessna Caravan needs a takeoff roll of 2,341 feet/714m. However, the actual requirements force a 3660ft/1116m runway. I didn't get the landing distance required, but I think it's safe to say that the landing would be shorter than the takeoff roll.
*the actual runway length required of a normal caravan is 2,055 feet/ 626m, and you can expect to rotate at 1,160 ft/ 354m.
*Some (incorrect) information was from: http://www.sgl.com/208.html
And more correct info from: https://cessna.txtav.com/en/turboprop/caravan
Please tell me if I should delete the first paragraph.