There are many swimming facilities through out the capital, many of which are open relatively late into the evening for those wanting to exercise after work. However, there are a number of (extra) rules of which you should be aware before you get into the pool. See here for a list of swimming pools in central Tokyo.
Swimming Cap Required
Unlike other countries where a perfunctory flick of the hair under the showers is enough to get you into the pool, in Japan hygiene is taken much more seriously. All swimmers are required to wear swimming caps and you will not be allowed into the swimming pool if you’re not wearing one.
Safety Check & Life Guard Change
This one really is quite unique to Japan. About once an hour everyone gets out of the pool and waits around for five minutes at the side. This can seem strange at first, especially when you’ve been swimming along lost in your own thoughts and all of a sudden find that you’re the only one in the pool. This short break is done so that the life guards can change shift and perform a safety check of the area. Often music will be played during this time, but given that you’re unlikely to hear this it’s a good to be aware of before you get a sudden tap on the shoulder from a lifeguard when you finish a lap.
No Swimming Equipment other than Goggles
Training equipment such as hard handle paddles are prohibited for the simple reason that you might hit someone coming the other way. Similarly, you will be asked to remove watches and locker key wristbands should be worn on your ankle.
Tattoos are taboo in Japan and many day onsen and private sports facilities will flatly refused entry to those with body marking. Public swimming pools are generally a little more lenient but you will be expected to cover any tattoos up with bandages or a rash vest.