Seasickness.

Only the very luckiest amongst us will be unfamiliar with seasickness. The three dimensional freedom of a boat's movement, as compared with the motion of land-bound vehicles, make this one of the worst forms of motion sickness, compounded by the sufferer's knowledge that there is no escape when at sea and the misery is likely to last for some long time.

'Mal de mer' is French, of course, and came into the English language in the 18th century. John Adams referred to it in his Diary, in February 1778:

"The mal de mer seems to be merely the effect of agitation."

The term had been in use in French for some time before that and is recorded with the 'seasickness' meaning by the late 16th century. It was also used in France to refer to another sickness of the sea, i.e. 'scurvy', and, according to the OED, there's a record of that usage dating from 1505.