Before i do anything else i am going to deliberately refer you away from this blog, to the wonderful Lunar Swimming entry on Randall Munroe’s What If blog, and please note that i asked that question. Please return for further discussion of the proposition.
All done? Okay, hopefully you noticed the bit that points out ‘The inertia of the water is the main source of drag when swimming, and inertia is a property of matter independent of gravity. The top speed of a submerged swimmer would be about the same on the Moon as here—about 2 meters/second’. So, if you can swim, you can load your muscles just as much as they are loaded when you swim on Earth, and swimming is one of the best forms of exercise there is. It develops all your main muscle groups while not straining your joints. The pressure of the water against you could also be useful for encouraging redistribution of fluids, if the low gravity isn’t enough to keep the fluids in our body where they ought to be (which it might be, and if it isn’t, the water pressure while swimming might not help - speculation here).
Let us come back in a bit to the much more intriguing fact you could leap out of the water like a dolphin, and the splashiness matter, and talk about how else so much water can be useful. And let us also come back to the point that water is heavy and it would take a lot of infrastructure to get the water for large swimming pools to the colony, and more infrastructure still to do the audacious things with them i am about to propose. That is all just a matter of how far along the development path of a colony something like this would make sense. Development timelines change a lot when people decide they want something to happen, thus the relevant thing is to talk about the neatest possibilities.
Water is excellent radiation protection, and it is transparent. The one-sixth gravity helps a lot in containing large masses of it. So, i propose putting a great big swimming pool right against great big windows, and making as much of the sides and bottom of the pool as possible out of transparent material like quartz, sapphire, or high-strength plastics. Really the only way to practically have large windows on the Moon is to put a few meters of water between you and the outside. ‘Practical’ is, of course, a relative term. I think it applies here because if you live in a place where going outside means hopping around awkwardly in a stiff space suit and still being stuck seeing everything through a glass pane and not being able touch anything, after a while i bet you aren’t going to be very interested in being outside. But if you stay inside, even the considerable effort made so far to create direct or reflected views of the outside will still give you only a narrow vista that is necessarily set a number of meters away. People are going to ache for a panoramic vista that can serve some limited substitution for the great outdoors left behind on Earth. So, this is the option. Until you can cover over entire craters in glass-and-water transparent roofs, which will make all the difference and i wouldn’t even recommend settling the Moon if that wasn’t completely feasible given enough infrastructure.
The pool sketched in the image is, of course, quite large. 57 m long, 7.5 m wide, 7.3 m deep, for a total of over 3 metric kilotons of water. So, since personally i don’t think there will ever be a good reason to mine water at the lunar poles for propellant on any significant scale, here is the good reason to mine that water. Hotels on Earth go to great lengths to provide guests with awesome pools, some of those even have large expanses of load-bearing transparent sections just because it looks cool. The justification here is essentially pretty similar.
As long as you stayed at least a meter or two from the windows, you could gaze out them to your heart’s content with no concern about radiation. Because you would be in that large body of water, the cone where there is so little water that radiation is elevated would be so narrow that the resulting dose is safe. Other tricks have been employed in this cylinder too - it is covered with plenty of regolith, and is under an overhang wide enough that not a lot of sky is visible if, for instance, your head is above water and you are quite close to the windows. A lot of the ground is visible, and scattered ionizing radiation will reach you from there, but that dose should be less than the dose from skywards. (I am going to need to seek numbers for that.) It seems to me that lunar residents are best left to judge for themselves how much time to spend in observation spots that come with a dose of radiation, and i imagine they will manage that without trouble. 10 minutes with a breathtaking view is worth a minor radiation hit a healthy person can manage without trouble.
And if you hanker getting moving, leap like a dolphin!!! To get up to a good speed at a good angle for an impressive leap, you want a pool this deep, and this long. Let’s review the figures for a fit swimmer with flippers - they could leap 2 m out of the water. Oh yeah! I’d be in that pool all the time. This is an environment where you could really tire yourself out with a good race without being painfully aware of how often you have to turn around because of the limited space. It is a place for epic splash fights because droplets would hang in the air 6 times as long. Once i detail the pool i’ll add a shelf where triple-decker chicken fights can be conducted, a suitable variety of diving boards, and 3 levels of pool-side lounging area set against its glass walls. The diameter of the cylinder that houses it actually now seems too small. It can only fit a non-springy diving board a measly 4 m above the water and you’d have to watch you didn’t jump too high and hit your head on the roof. Clearly this pool requires a spring board at least 15 m above the water, and one would need to expect that jumping off it would launch you another 4 m higher. The architecture will need to be reconsidered…
In sum, exercise on the Moon that would genuinely load your body just like on Earth is entirely possible and would be super super fun. This i think is enough for us to not concern ourselves about muscle and bone wasting as long as people are willing to swim several times a week, which really doesn’t seem like it will be a hard sell. There are other possibilities of course, which we can explore once detailed large models of the Gallery are done, like the simple but effective option of having no stairs anywhere and simply jumping between floors. And further out still, with a covered crater model, we can talk about artificial bat wings that allow you to actually fly. Yes, i’m serious.
Oh, i almost forgot, large amounts of water are also excellent heat sinks, greatly reducing the demands of thermal control. Yeah, that’s kind of boring, isn’t it… forget i mentioned it…