In July 2016, ESA astronaut Matthias Maurer took part in NASAs Extreme Environment Mission Operations, or NEEMO a 16-day underwater sortie to enact a mission to Mars and test equipment for astronauts.
NEEMOs underwater habitat off Florida acts as makeshift a space base for astronauts to make regular waterwalks in full scuba gear. Both underwater missions plan sorties for the astronauts to simulate spacewalks.
For NEEMO 21, a rotating crew of four complemented Matthias and NASA astronaut Megan McArthur who tested new techniques, operation procedures and technology.
Connect with Matthias: http://matthiasmaurer.esa.int
More about NEEMO: https://www.nasa.gov/mission_pages/NEEMO/index.html