Technically, this isn't really a lido: it's just part of the Serpentine lake that's been marked off with a line of buoys, rather than an outdoor swimming pool. For instance, rather than having tiles along the bottom and sides, you're actually standing on the sand at the bottom of the lake, and there's seaweed drifting around. I stood on something else underwater, which felt like wood, although it's odd that it wasn't floating. The water has been rated as very clean, but it's not clear; if I put my head underwater, I could see as far as my hands feet (when I fully stretched out my arms and legs), but apart from that it was all just a green void (algae). Meanwhile, there were ducks swimming around near the edge, and sitting on the buoys. So, it's a bit different from a normal swimming pool! The depth varied; basically, it got deeper as you moved away from the edge, so it was waist deep where I entered the water (via the ramp), but further out it was too deep for me to touch the bottom while keeping my head above the surface.
Since this was outdoors, the water obviously wasn't heated, so it was a bit colder than an indoor swimming pool. However, it wasn't too bad; I'm not sure of the exact temperature, but it was just "Ooh, that's a bit chilly" rather than "I have icicles on my fingers!" We couldn't jump in, since it wasn't deep enough, so the next best thing was to stand in place and then crouch down suddenly, to get the shock over and done with. After that, it was fine as long as I kept moving, and some of the time I actually felt quite warm. My normal swimming style involves putting my face underwater during breaststroke and front crawl, but my first attempt at that felt very cold, so I decided not to try that again. I then changed my mind a bit later, once I'd acclimatised to the water, so I was happy to swim completely underwater by the end.
Apparently the lido is 100m long, so 10 lengths = 1km. One difference from being in a pool is that you can't push off from the side, so I found it a bit more challenging to maintain my speed for an entire length. Also, there's wind and a slightly choppy surface to deal with; not much of a current, but at times the water level did bob up and down. Also, it felt a lot more spacious than usual: there were no lanes marked out, and even with about 20 people in there (at its busiest) we could easily avoid each other, unlike an indoor pool where I'm always trying to avoid collisions.
The cliche about living in a big city like London is that you tend not to bother doing "tourist stuff", because you can do it anytime, so it's easy to delay trips to museums etc. This is slightly different, since tourists probably wouldn't do it either, but I've lived in London for 13 years and this is the first time I've been there. Afterwards, we had a picnic on the grass, which was very pleasant, and it made me remember that there are places of tranquility in the midst of the concrete; you just need to look for them.
So, all in all, a good day, and I certainly intend to do it again. I also think that I'll sleep well tonight!