"The horse drinks water."
Translation:Il cavallo beve l'acqua.
The verb you need to change is "bere", which means "to drink" (infinitive form). It is irregular because it changes from R to V (you don't just change the ending). The form you use depends on WHO is drinking (the subject).
I drink= (io) bevo
you (informal) drink= (tu) bevi
he/she/it drinks OR FORMAL YOU (Lei) drink = (lui/ lei/ Lei, or some less common forms) beve
we drink = (noi) beviamo
you (plural) drink = voi bevete
they drink = (loro) bevono
Hope that's clearer :)
Italian almost always uses the definite article "the" with a noun, whereas in English we tend to only do this when talking about a specific example of the noun. I think your sentence may be acceptable (you'd have to double check with a native speaker) but it's always safer to use the definite article in Italian.