This is actually a non-trivial question but it is not open to opinion, it is simply a matter of creating a heat budget
. First you need to throw out all data from cars, a car in the atmosphere is in a completely different environment
than a boat in the water
Lets start by simplifying and considering only a pure black boat (hull and deck) and a pure white one. We'll further assume that the black hull has an albedo of 0, that is that it absorbs all solar
radiation and reflects none (a perfect black) while the white one has an albedo of 1, it reflects all radiation which reaches it and absorbs none. We'll further assume that both boats are sealed boxes with no air exchange with the atmosphere.
There are five primary mechanisms by which heat is exchanged with the boat. The first is short wave solar
radiation (sunshine). The second is long wave radiation (heat radiating away from the boat as IR waves). The third is evaporative cooling (latent heat exchange) off the skin. The fourth is conduction between the boat and the water. The fifth is conduction between the boat and the air. We will ignore convective exchanges, rain cooling and such.
During the day, SW_rad dominates over LW_rad while that reverses at night. During the summer in the Sea of Cotez that SW_rad term will peak around 500 W/m^2 at mid-day but average to about 30 W/m^@ over 24 hours. Evaporative cooling is negligible. LW_rad will depend on boat temperature. Conduction will vary depending on the relative temperatures of the boat, the air and the water.
Letting both boats start at the same temperature in the same conditions, the white boat gains no heat from SW_rad while the black gains 30 W/m^2 on average. LW_rad is initially the same but increases linearly as the black boat increases in temperature. Thus as the black boat gets hotter it also radiates heat away faster. Temperature exchange with the air and water scale the same, as the black boat heats up more from the sun it exchanges heat with the air and water at a rate that scales with the temperature difference.
So if the air and water are hotter than either boat and the black boat is hotter than the white boat, it will gain heat from the air and water more slowly. the converse applies as well. If the air and water are cooler than the boats and the black boat is hotter than the white boat, it will cool off faster.
I've run a quick simulation assuming air and water conditions as taken from 1 AUG 2012 to 1 SEP 2012 from the COAMPS reanalysis and find that for totally sealed boxes the internal temperature in the black box is ~2C warmer (on a daily average) than in the white box. The biggest difference occurs during the day with nights being generally less than ~1C difference.
I did not run the numbers assuming airflow but it is reasonable to assume that a well ventilated boat will see little difference in interior
temperature, surely not enough to dictate your paint
Take with a grain of salt
, just some first order calculations from a sailor with a Ph.D. in ocean physics and some time to kill.