I saw a documentary awhile back and they were profiling a certain species of monkey which regularly swims underwater. I can't recall which species it was but they said it was rare for primates to do this - diving under the surface, holding ones breath, and swimming completely submerged. 

I'm wondering which type of monkey this was and what other primates, besides humans, are known to engage in underwater swimming? Is there a reason this behavior evolved? My guess is that it has to do with foraging for food in mangrove type environments and possibly for avoidance of predators.