You don't need other people to swim, but a Microsoft Research project can spice up your swimming routine by putting you in or pitting you against virtual teams. Redmond's research division is working with a team of researchers from Korea Advanced Institute of Science and Technology (KAIST) to develop SwimTrain. It's a system consisting of an app, a waterproof case for your smartphone and a pair of underwater headphones that provides sensory/auditory feedback. SwimTrain puts you in a team of three swimmers -- if you're competing, it tells you how you're doing against your two virtual opponents. If you're in the same team, it helps you maintain your strokes so you can keep up with your virtual teammates.
The KAIST team created SwimTrain, because they wanted "a group fitness swimming game that allows a group of people to perform mediated synchronous interactions over a virtual space." When they interviewed the 11 study participants who used the prototype they made, they found that the system "enriche[d] the social experience of swimming, motivate[d] swimmers to greater levels of exertion and allow[ed] swimmers to establish a strategy to win the game irrespective of their relative skill level." Microsoft Research didn't say whether we'll ever get the chance to use the system, but you can know more about it through KAIST's paper and the video below.