Can someone please clarify this for me?

In "The Secret Life of Walter Mitty" written by James Thurber, in the first paragraph is Walter Mitty commanding a boat or plane or what? (Walter Mitty is the commander)Can you explain what exactly a “navy hydroplane” is? I looked it up in the encyclopedia but that just confused me more. You can find the short story at a href="http://www.all-story.com/issues.cgi?action=show_story&story_id=100" rel="nofollow"http://www.all-story.com/issues.cgi?acti…/a -I am only concerned with the first paragraph. Thank you.