Quite a few Navy astronauts have flown in space. It'd be interesting to see if there is any difference in space sickness rates between them and other astronauts. Given the time it takes to go through astronaut training (several months to years depending on the crew position), it's possible any acclimation to motion would've been lost by the time they fly.

Then again, perhaps severe sea sickness would be a form of selection mechanism for sailors. Those who tend to puke their guts out probably wouldn't stay in the Navy very long, so they wouldn't have the opportunity to become astronauts.

Off hand, it seems that aviators would have the most experience at dealing with motion sickness. However, quite a few astronauts (including aviators) experience space sickness, even back at least as far as the Apollo days. Here's a good article on the topic:

One medical problem that astronauts in the Gemini program appeared to have avoided was space motion sickness. "It was never reported in the Gemini crews," said Coons. "I don’t know whether they didn’t have it or whether they had it and they didn’t want to admit they had it. But my impression was that we would have learned about it sooner or later from one of them, if it had been the case." What worried the NASA doctors most was the possibility of someone getting sick during an EVA. "We thought a lot about what would happen if a person became nauseated and needed to vomit inside a space suit. We were terrified of that."



In retrospect, it seems somewhat puzzling that the Gemini astronauts were spared—if, indeed, they were. It’s now known that most astronauts are afflicted by space sickness to some degree during the first few days in flight. The fact that the Gemini capsules were so cramped might have had something to do with it; later spacecraft, such as Apollo, Skylab, the space shuttle and the space station, provided crews much more room to move around, which increases the probability of disorientation and space sickness. "As long as they were strapped in, it minimized it," said Craig Fischer, Chief of Space Medicine and Health Care Systems at the Johnson Space Center, a veteran of the early days of the space program.

However, even the astronauts who performed EVAs in Gemini did not report problems with space sickness. "I don’t think there was ever any mention of it and I don’t know whether they ever had it or not," said Coon. According to Fischer, however, one Gemini spacecraft came back with a dark stain on the console. Medical director Chuck Berry had it analyzed. "Berry was worried it was blood," said Fischer. "We saw on the test that it was chocolate pudding. I have no idea how it got there; I don’t know whether that was the first documented episode of pitching chocolate pudding."

The first time the issue really attracted the attention of the media and the public was during Apollo 9, when astronaut Russell Schweikart abruptly vomited twice. This prompted commander James McDivitt to request a private conference with doctors on the ground. "Rusty Schweikart really got sick and the cat was out of the bag," said Fischer. "When you went back, you found out that there had been some episodes before that, but they never made a point of it."

The private medical conference on Apollo 9 raised a considerable fuss among reporters, who believed all aspects of the mission should be public knowledge. But the astronauts were pushing for greater security for their medical information. "In those days, everything that got called down became sensationalized," said Fischer. Private medical conferences—which are now a standard feature of space shuttle and space station missions—were a way to ensure that "it’s not going to become part of the national press when they have a bowel movement, if they vomited, if they didn’t get a good night’s sleep. Now, it’s between the flight surgeon and the crew. The agreement with PAO [Public Affairs Office] is, if someone really gets in trouble, gets sick up there, it goes in the open, but that’s only if it’s a really significant impact to the mission. If the flight surgeon doesn’t feel that there’s any significant impact, he would not relay it to anybody else because he alone is responsible for the crew conference."



Here's some more info:

Vestibular disturbances with nausea were noted by Soviet Cosmonaut Titov during his 1-day-Vostok 2 flight on August 6, 1961, and by the crews of other later Soviet flights. No astronauts had experienced any motion sickness symptoms until the early Apollo experience. In retrospect, however, the anorexia and reduced caloric intake observed on certain Gemini and later Apollo flights, may have been, in fact, early symptoms of vestibular disturbance.



Apollo 8 and 9 especially were plagued with vestibular problems: five of the six crewmen developed stomach awareness, three of the six, nausea, and two of these six proceeded on to frank vomiting. In Apollo 15 and 17, three of six of the crewmen also experienced stomach awareness. The flight plans of Apollo 8 and 9 required that the certain crewmen leave their couches soon after orbital insertion. All three Apollo 8 crewmen noted some motion sickness symptoms (stomach uneasiness or awareness, nausea, or vomiting), confined generally to the first day of flight. There is some confusion concerning the etiology of the Apollo 8 crew’s symptomatology, since the Commander felt that a viral gastroenteritis accounted for (or aggravated) his symptoms. In Apollo 9, the vestibular disturbance lasted for a considerably longer time, and in the case of the most severely affected crewman, necessitated a postponement of the flight plan. And thus an additional problem area was introduced into the American space experience. This disturbance, which had long plagued the Soviets and which had been predicted in the early 1960’s as a probable effect of weightless flight, had made its belated American debut. Its late appearance was probably related to the relative immobility of the crews in their spacecraft during the Mercury and Gemini flights and the absence of any rotation of the vehicles themselves.



They aren't kidding about the limited mobility in Mercury and Gemini spacecraft. I looked into the cockpits of both spacecraft last week at the NASM. Mercury capsules were incredibly cramped. The usable volume of a Gemini spacecraft looks no bigger than a phone booth and they flew missions as long as 2 weeks in those. Apollo was much larger than Gemini and it rotated a lot during the flight to even out the heating.