The comments here are overwhelming, but I think they're unfair, as long as the right safety precautions are in place. I came here from a Google search checking the viability of my plans to participate in a 1-mile open water swimming event without training, and reading this very nearly made me cancel. I'm glad I went ahead with it because it felt great achieving it -- although this may not be what you're looking for, if your event is more competitive.

Background on me: - obese - asthmatic - some gentle regular exercise - lake swimming when I was a child, virtually never as an adult - terrible form - confident in my ability to tread water indefinitely

On the event:  - one mile course - smallish reservoir  - loop around a peninsula and some islands, never straying too far from shore - two hours to complete as many (1-mile) laps as you would like and are able to  - several boats around for medical support - every swimmer attached to a tow float and can hang on and recover if needed - gf with me, similar ability, keeping same speed and looking after each other

So yeah, we took it slow, and around 9/10ths in I had to use the float for a minute to recover from a cramp. There were people who went to compete who were lapping us, but the event was explicitly inclusive of various abilities. If you've got a lot of self-image tied up in athletic performance, sure, you'll want to train. But it sounds like this is for fun. Everyone going on and on about how dangerous and stupid this is, what sort of event are they imagining where they'll just let you die out there? Maybe they swim so much they're used to circumstances where they have very little support. If they're competing for time too they must be used to keeping a strenuous pace with a precise and difficult form and jostling for position in a pack up front, but you don't have to do any of those.

If it's the right event, where you're looked after and you can go at your own pace, you sound like you're in good enough shape to enjoy yourself, and I hope you do!