Is it okay to leave cooked octopus in water till next morning?
I've realized that the price of mistake is not the high anyways so I've just divided cooked octopus to two parts: one left in water, another one not. This morning I finally have an answer!

It's better to take cooked octopus out of water but it won't be a tragedy if you didn't.

While both meals were tasty, octopus left in water was slightly slimy - it was a very subtle difference however noticeable.