As seen here, seaplanes have these, or they'd be at the bottom of the sea
[DOC] [TLE] PA-12 Seaplane Door - need advice & ideas - SuperCub.Org [PAR] The Door that is on her now is the standard PA-12 door with the hinges ... be 
handy to use as a latch mechanism at the bottom to hold the door .... STC---yes I 
believe you do---I bought my plane 12 with sea plane doors right and left.