I have a friend who has seen two of them in Alaska. Apparently they have thick, obtrusive bracing between the two wings inside the cockpit along the ceiling. He says they like them a lot. I would think that the water would corrode the rear engine and prop like crazy, but my friend says no. I have never seen one myself, and do not have pictures.
The best source of pictures of Skymasters is
www.skymaster.org.uk. This site has a huge number of pictures on it. I don't know if their are any float equipped 337 pictures, but you might email Peter the webmaster and see if he knows of any.
Kevin