Learn Swedish: Gå ner till havet!

This is the core of positional vs. directional adverbs! Because you are moving towards a destination (going down to the sea), you use the directional form ner. If you were already hanging out at the beach, you would use the positional form nere (for example, jag är nere vid havet meaning I am down by the sea).

Is Gå the infinitive form or a command?

It is a command (the imperative form), telling someone what to do. For short verbs that end in a vowel like gå (to walk or go), the imperative form is exactly the same as the infinitive form.

Why does hav have -et at the end?

Hav (sea) is an ett word (ett hav). To say the sea, you add -et to the end, making it havet.

I have seen the word ned before. Could I use that instead of ner?

Yes! Ner and ned mean exactly the same thing and are both directional adverbs. Ner is just much more common in everyday speaking and writing, while ned sounds a bit more formal.