I guess a seaplane can't veer from the moment it touches the water until it slows down enough to do it without capsizing. Wouldn't it be fair to consider that it is restricted in its ability to maneuver during this period?
Nope. Basic right of way pecking order seaplane always gives way to sailboat.
"Restricted in Ability to Maneuver" means the vessel is displaying shapes or lights to indicate they are restricted. This is defined in the COLREGS. There is no interpretation of a vessels ability to maneuver, either they are displaying these shapes or not.
Now, if a seaplane is coming in to land and a vessel changes course as to interfere with the course of the plane, you have a different scenario. But, the seaplane is required to keep clear and plan its approach such that it will not force another vessel to change course because of its actions. In Victoria Harbor on Vancouver Island the seaplanes come in and out of the harbor all the time. There are very defined traffic lanes for seaplanes and water vessels, and they are very close. Sometimes can be scary because they will bank hard and descend very quickly, in very close proximity to vessels in the water. You have to stay in your traffic lane and trust the seaplane will do the same.