We saw on the SpaceX Demo 2 splashdown livestream that various clearly-non-mission-associated civilian boats appeared to approach the Crew Dragon within 100-300 ft. Has this happened on any previous water landings of spacecraft?
Most of the Apollo missions and some of the Mercury and Gemini missions landed far out in the Pacific Ocean. The rest of the Mercury and Gemini missions, plus Apollo 9, landed far out in the Atlantic Ocean. The mission that came closest to land was Gemini 3, 110 km off Middle Caicos Island, and that was only because it missed the landing target by 84 km.
Also contributing to the lack of crowds is the fact that all prior missions were recovered by aircraft carriers, which tend to discourage civilian onlookers.