Globe Swift (Bill Lyons) on amphibious floats
Grumman J2F Duck (Paul Clawson or SwingMan) many pre-WW2 paints available
Grumman G-21 / JRF Goose (Bill Lyons, Mike Stone) many pre-WW2 paints available
Grumman G-44 / J4F Widgeon (Jerry Beckwith, Joe Binka) both in pre-WW2 USCG livery
Shavrov Sh-2 (Alexander Belov)
Sikorsky S-38 (George Diemer)
Sikorsky S39 (George Diemer) as Willy suggested
Vickers Type 54 Viking IV (Ted Cook)
Vought OS2U Kingfisher (Tim Conrad) many pre-WW2 paints available
The S.38, Goose and Widgeon are twins, but they're small twins. The others are all singles.