Louis Eilshemius

Apparently he really did sail out to the South Pacific and came back with a stack of scenes like this one. They say almost nobody took him seriously for decades — until Marcel Duchamp, of all people, started calling him a genius, which only half stuck.

They say he spent decades loudly declaring himself a genius to anyone who'd listen and got nowhere — until Duchamp, of all people, picked him out of an exhibition and started championing him.