The largest source for freeware boats is probably
FS Shipyards. You'll have to look carefully though since they're not all FSX compatible.
http://fs-shipyards.org/There are a few links to some freeware boats listed in the
Marina section of
The Moorings here at RTMM. Just look for it under
Site Map.
I know of at least three freeware a/c that are available in multiple configs (wheels/skis/floats/tundra). One is the
Corben Baby Ace by Craig Richardson and there is the
Stinson 108 by M. Allen Richards. There's also a series of
Early Fairchilds by James Hefner. Check the usual sites (SimOuthouse, Simviation, AVSIM, FlightSim). I often find it easier to search under the author's name, rather than the aircraft's, since some sites have weird parameters for their search engines.