It looks fine to me - no soft mushy dark patches. It's growing - that's why you see wrinkled bits, like a newborn baby. It will eventually 'iron' itself out, ha ha. Just keep doing whatever you are doing.

Thanks for the answer!

Do you know what it is?
Have you repotted it recently? What compost is in? I have not seen one do this before. Have you checked the roots for mealy bug? It might be an idea to PM Hywel - might be slow replying as he is a bit busy for Goy at present.
Don't worry about the water not reaching the roots - it should happily go for ages without water.

I don't know what kind of cactus it is. It was repotted in May because it was outgrowing the small pot it was in. I have not checked for mealybug but in May the roots looked very healthy. Water drains straight out of pot in a few hours.

It never used to have these wrinkles when it was watered with tap water. It was still kept outside.

When it is watered, the the soil should be thoroughly soaked. That's hard to do in a pot with the rain, unless it's a three hour downpour! So, it might just be lack of water, though the fact that it takes hours to drain isn't good news. A good soil for cactus should drain from saturation to field capacity--where gravity can no longer pull water out--in a half hour or less. I would gently pull back the top cm or two of soil, so that you can see the root crown more clearly, and look for black mushy spots, as Bathgate says. If those aren't present, I would give it a good soaking, and see if it plumps up over the next few days.

I will give it a good soak. I ment the soil dries out completely in hours :)

Oh! Almost certainly lack of water, then. Cactus soil needs to be able to hold some moisture for a few days, so the roots can grow a little and form root hairs. One reason why cacti are rarely found in pure gravel.

The cactus is fine now.

Excellent!