The Onzin Institute in Amsterdam conducted a four-year study called "Environmental Effects on the Virility Index", and the results astounded even the program's Director, Dr. Hijgen Krachtig.
While part of a larger project which attempted to measure human sexual drive, ability and potency on a standardized scale, called the Virility Index, the study itself took a group of 2,000 Dutch couples and measured the subject's reaction to several stimuli using a number of sensors, including heat-sensing video cameras, a blood-enzyme monitor, multiple skin and scalp sensors, and something called a "self-calibrating turgidity anchor". The second part of the study measured the couples during actual lovemaking, and then the results of both experiments were collected and cross-referenced with the subject's personal data to provide the results for both individuals, and couples.
People who took part in outdoor leisure activities had far higher scores on the index than those who did not, and those who listed "boating/sailing" among their top three activities scored, on average, nearly double that of any other group. The same thing with occupation: Those whose job was listed as "maritime" scored much higher than those who worked on land.
"We think that it may have something to do with the inner ear, or it may be due to the strengthening of core muscles as the body compensates for the rocking motion found at sea," said Dr. Krachtig. "Although one of my colleagues has suggested that maybe sailors are just better used to making love in uncomfortable environments." A follow-up questionnaire did not find any significant difference between the displacement or type of craft sailed, and the individual's or couple's virility index score, indicating that it is not the size of the boat which makes the difference, but rather the motion of the ocean.
(Ah ha! So now we know!) Full article: Yachting Monthly.