Oriental Trading Company has TONS of ducky stuff. Also check out the site www.birthdaypartyideas.com - there is a kids section, and you can click on the rubber ducky theme and get tons of ideas.
I had a duck theme for my sons 1st bday, 5 years ago. I got tons of rubber ducks for our (backyard) pool, kids sun glasses that looked like 2 ducks face to face, and duck partyware.
I got clear plastic tumblers, and made blue jello in them, and put whipped cream on top, then added candy rubber ducks (Wilton makes them, available in the cake aisle of most craft stores) on top. I took my big, glass punch bowl and put lemonade with lots of blue food coloring (turned out turquoise) and a rubber duck floating on top (you could also use blue Kool Aid of course). Had a kiddie pool with rubber ducks in it, with colored dots on the bottom like they do at carnivals, and kids got to pick from the prize bucket that coordinated with the color on the bottom of the duck.
It is a great theme, have fun with it!