I have asked myself this question multiple times. "What aircraft if any, is the sea duck based off of?" Every time I give myself the same answer. The Grumman Goose. I think it's very similar to the sea duck because its amphibious and can also land on land, it also has a very similar look. It was also produced around the same time period.
Here's the Wikipedia page
http://en.wikipedia.org/wiki/Grumman_G-21_GooseI know it doesn't look exactly the same but it's similar.