We use swimming pools fairly often but our kids always want to go swimming.
A friend of mine uses a national gym chain that he is a member of.
Also we have used campsites just to empty the loo and have a shower, the last one cost £5 for 30 mins.
I just pick them at random and rock up and ask them if we can.
Most have no problem.
I have also just walked into a campsite before and used the shower after parking up the road, shhh!!
Also have used public and outside showers that they sometimes have at public loo blocks, on the beach, for example.
However as Neil says a solar shower or just a black container works fine too.
Mind you we have a shower in our motorhome but we don't use it very often, only if away for a week or more and then the 4 of us use about the same amount of water as 2 washing up bowls when we shower.