Is it common in Japan to share food?

for example, order only 1 yakisoba or only 1 okonomiyaki as snack and eat in 2 in a restaurant? and then pay only for one off course..
I'd also like to know is water is always free , at restaurant and chains like Yoshinoya and mac donald's

for example, order only 1 yakisoba or only 1 okonomiyaki as snack and eat in 2 in a restaurant? and then pay only for one off course..

Sure, its common to share these sorts of things. Of course it depends on the type of restaurant and the people you're going with. But shouldn't your split the bill rather than just paying for one item?

I'd also like to know is water is always free , at restaurant and chains like Yoshinoya and mac donald's

Yes. Places like yoshinoya serve free tea as well.

Actually, while I'm pretty sure Mac Donald's offers you free water, I've never seen it served. I think you have to go to the counter and request for it.

As for food, most restaurants expect customers to order at least one item per each person, but sharing is no problem and very common. Often the waiter will offer to bring you a smaller plate for you to help yourself put your portion on. If not, just gesture a round plate shape and they will most likely bring some for you.