Where is the landing distance for seaplanes specified?
is there a specified distance for the landing areas of seaplanes?


A cautionary note: As @MikeSowsun said, the required distance to land a seaplane is MUCH shorter than that required to takeoff. For operational planning purposes, what you should be interested in is takeoff distances. My offhand guess is that landing distances are less than half that of takeoff runs. At least that was I think true of the old J3 floatplane I used to fly.

Now I assume you're including floatplanes as seaplanes. There are relatively few hulled seaplanes these days as compared to floatplanes, and compared to the numbers of the wheeled configuration of a given aircraft for which floats are available, the number of floatplanes is relatively small, which may account in part for the POH (pilots' operating handbook) for aircraft usually not providing performance data for float configurations. There are also differences in performance depending on the make and model of the floats being used.

All that said, you can often find performance details for current production floats on a given aircraft on the float manufacturer's website. For example, http://www.wipaire.com/aircraft_page/cessna-caravan-series/ gives that information for the Cessna 208 on Wipaire amphibious floats. In this instance, the takeoff run on water with the Caravan's small engine is 2341 feet, takeoff over a 50 foot obstacle 3660 feet. The same manufacturer's figures for a Cessna 206 on their amphibious floats is 1770 feet takeoff run, 2850 feet over a 50 foot obstacle.


  just making the landing area by guessing?


Not by guessing, but looking at the intended area of operation in the light of past experience and judging whether they can takeoff from that area.