> As part of the Astronaut Candidate training program, candidates are required to complete military water survival before beginning their flying syllabus, and become SCUBA qualified to prepare them for spacewalk training. Consequently, all Astronaut Candidates are required to pass a swimming test during their first month of training. They must swim 3 lengths of a 25-meter pool without stopping, and then swim 3 lengths of the pool in a flight suit and tennis shoes with no time limit. They must also tread water continuously for 10 minutes wearing a flight suit.
Source: https://www.nasa.gov/centers/johnson/pdf/606877main_FS-2011-...
I'm actually Canadian and when I looked into applying for CSA, they stated the requirement was a strong swimmer. They did some drills in the Atlantic.