"He likes water."
Translation:Er mag Wasser.
In German, the endings of verbs change depending on the person you're talking about. Some remnants of this can still be seen in English: you say "I like" but "he like-s" with an additional "s".
The forms of "mögen" (to like) are:
ich mag (I like)
du mag-st (you [singular, familiar] like)
er/sie/es mag (he/she/it likes)
wir mög-en (we like)
ihr mög-t (you [plural, familiar] like)
sie/Sie mög-en (they/you [singular & plural, formal] like)
In your sentence, you're talking about "er" (he), so you have to choose "mag".
Unfortunately, like all the other modal verbs (e.g. can, must, may), "mögen" does not behave like a normal, regular verb. There is a vowel change in the entire singular (ö->a) and there are no verb endings for "ich" and "er/sie/es". A normal, regular verb looks like this in the present tense:
mach-en (to make, to do)