Entry 1 Water reflects sun back at you
Open water reflects a significant amount of additional ultraviolet light back upward, meaning sun exposure on a boat is often considerably higher than the same amount of time spent on land would produce, even when it doesn't feel that way in a breeze.
Entry 2 Wind hides how hot it actually is
A steady breeze while underway makes heat feel far less intense than it is, which often delays the obvious signs of dehydration or overheating until they're already fairly advanced, especially once the boat slows down or stops and the cooling effect disappears.
Entry 3 Reapplying matters more than the initial application
Sunscreen applied once at the dock in the morning is rarely enough for a full day outdoors, particularly with water spray and swimming both washing it off faster than a typical land-based day would.
Entry 4 Questions people ask
Why does sunburn seem worse after a day on the water than a similar day on land?
The combination of reflected sunlight off the water and a cooling breeze that masks heat both contribute to more sun exposure than feels apparent at the time.
How often should sunscreen be reapplied on a boat?
More frequently than on land, especially after swimming or heavy spray — checking the product's own reapplication guidance is a reasonable baseline.