Learn Hardcore Norwegian: De ser en svær båt.

When it is written as De with a capital D, it can also be a very formal way of saying you, similar to old-fashioned polite forms in some European languages. However, that usage is rare in everyday modern Norwegian.

So in a learner sentence like De ser en svær båt, you should usually understand de as they unless the context clearly shows it is the formal you.

A few useful points:

de = they
De = sometimes formal you (old-fashioned or very formal)
In most modern writing, you will often just see de

Why is ser the form used here? Shouldn’t it change depending on who is doing the action?

No. Norwegian verbs do not change according to the subject the way English verbs sometimes do.

So:

jeg ser = I see
du ser = you see
han ser = he sees
de ser = they see

The verb stays ser for all of these in the present tense.

This is much simpler than English, where you have I see but he sees.

What form of the verb is ser?