I think it's nadar because "va al mar" means he is going to the sea. Therefore, he would be going to swim.
Answer:
We have homework in class. (little)
Explanation:
Tenemos is the nosotros form of tener. so you could say "nosotros tenemos" but "tenemos" works because tenemos is the only way to say the verb tener in the nosotros form so people get what you mean when you just say tenemos