I'm a bit biased, it's totally swimming! Heh, well, I suppose for Winter Olympics, curling most definitely fits the bill, but you know, a lot of curlers have problems with their knees after years of playing the sport (and many get knee surgery). If you take care of your body well, and don't have any stupid-and-most-often-easily-preventable accidents (crashes) with others in warm-up, then it's really hard to injure yourself in swimming. You've gotta have the right technique, though, 'cause some techniques out there can 'cause long-term strain on your body that shouldn't be there if you're doing it right. Drowning is not a concern Olympic swimmers have - they can easily float spending little to no energy to do so, being in complete control of the water around them (and it's a pool! Clear bottom, walls, etc). It's rare, but it has happened where someone has slipped on deck and injured themselves.
Swimming is probably one of the fairest sports (if not the fairest sport) in the Olympics - everyone has a separate lane (no one can affect your race by crashing into you), there's electronic touchpads on the walls to clock the time (and in the blocks for dive starts... which is really useful for relay transitions), there's a no-false-start rule (because, like in the skating events, they used to have the one-false-start-is-ok-but-then-everyone-is-on-the-hook-for-the-second-false-start rule, until people started setting off the first false start to put everyone on edge, so now it's just, no false starts at all, if you false start, you're disqualified), and the only real judging is for whether the stroke is done right (which is hard to get wrong when you've trained it so much; for example, you can get disqualified if you bring your arms out of the water separately during a butterfly race, or don't touch the wall with both hands at the same time during a breaststroke or butterfly race, or take multiple freestyle strokes into your backstroke turn, etc. But it's really hard to get disqualified for stroke violations in freestyle, 'cause as its name implies, you can kind of do anything with it).
Curling has a tad more human/other error involved than swimming, 'cause sometimes a rock will pick on debris on the ice and head a different direction, or the timekeeper will forget to start/stop a team's clock on time. But in swimming, there's nothing to interfere with a swimmer's race but themselves, and the judging is minimal (it's obvious if the swimmer misses the wall on a turn or makes a stroke violation... but those things are pretty much non-existent at the elite level... and as I mentioned before, the starting blocks at recent Olympics have sensors in them which help notify of disqualifications in relays for a swimmer diving before their teammate touches the wall). The lanes are equal and the water doesn't change from race to race (not like snow or wind) (well, if the pool hasn't been built in recent years, the outside lanes closest to the wall might get some wave-feedback, but the latest Olympics have built brand new pools that have edges that eliminate those waves... and newer laneropes also cut down on waves).
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