A predicate is the part of a sentence that contains a verb wich tells something about the subject.
Now, in simple present or present progressive the formula is: subject + verb+ complement; the verb is the predicate. Taking the first three example, the subject is circled and the verb verbs comes after.
Answers are underlined:
The West Indian Manatee is large a sea mammal.
Manatte rest just below the water's surface.
Sea grass is one of their favorite things to eat.
In the last one it can't be eat the predicate because the predicate refers always to the subject, and 'sea grass doesn't eat'. "To eat is a complement"