time I jumped in a pool with the intention of preparing for a triathlon I swam 2 lengths and then asked the lifeguard what the length of the pool was. I remember being not happy to hear it was only 25 yards per length. I think it took 6 weeks before I could swim 300 yards without stopping.
I saw that a lot with runners who joined the triathlon club when I was in grad school. They would hop on a bike and be able to survive (or even thrive) on a longer ride with more experienced cyclists and expect the same when they jumped in the pool. They'd get to the other side and then grab the wall and wonder wtf just happened to them! It was amusing.
Oh, "without stopping" is definitely not an option! If 400 m were possible (and you're both probably right that it's not), I think I'd have to use a wetsuit and stop to tread water a few times. I'd have to try it in a pool first so I don't drown in open water!
I wasn't always a bad swimmer. When I was a pre-teen, I was selected for the city swim racing club but my family wasn't the 6am practice type and I eventually forgot most of what I knew.
There was an 85-year-old racing last night! I think he did 2 km. Wow.