Magic mushroom spores do not exactly grow in water, but they are typically preserved in a liquid environment, until they are being used in a proper substrate, where they will develop into a new mycelium.
Mycologist and other people who grow laboratory grade mushrooms for studying them use spore syringes, as they are very convenient. It is also legal to purchase them, as magic mushroom spores do not contain psilocybin, in this stage of their development.
Magic mushrooms grow in the wild but only in certain areas, so not everyone has access to them. A mushroom spore syringe is the best alternative, as you can use it to “sow” your substrate and grow your batch of mushrooms.
Spore syringes are filled with sterilized water that has been boiled a few times, to eliminate bacteria that may contaminate the spores. This liquid environment is ideal to preserve magic mushroom spores, as they will not germinate there, as long as there are no nutrients in the water.
However, you should make sure you purchase spore syringes from reputable sources such as Mush Love Genetics to avoid unpleasant surprises, such as contaminated spores or even germinated spores that could bring you legal problems (germinated spores do contain psilocybin!).