My wife does triathlons all the time and she signed me up for a sprint (400m swim - 5k run - 20k bike) last year. I grew up swimming in a pool but never consistently or over any sort of distance. My preparation for the swim was to hop in the lake half an hour before the start of the race and swim out to a platform and back (about 50m). It was really windy that day so the water was very choppy and that 50m was now the longest distance I'd ever swam in one trip.

Here's how it went down: We were in the second heat of four separated by 5 minutes. The gun went off I got past the dock and began getting slammed in the face with water (Power Tip - Open bodies of water != pools) I'm not even 75m in and I hear the next heat go off and the final heat by the time I'm about 150m. I was passed out by some kids and had resorted to the doggy paddle at one point. There were at least two life guards eying me the whole time just waiting for me to go under. I think the only thing that got me back into the beach was the current produced by the last heat passing me. My time for the 400 m swim was around 33 minutes and just for some perspective, the kids were finishing in like 16 minutes and My wife finished around 20.

TLDNR: I learned during a triathlon that I can float but not really swim.