Learn French: Mon chef voit le marin amarrer son navire. - My boss sees the sailor mooring his ship.

This is the standard structure for verbs of perception like voir (to see), entendre (to hear), and sentir (to feel/smell). To say you see someone doing an action, you use voir + the person or object (le marin) + the infinitive verb (amarrer). French doesn't use a present participle (an "-ing" equivalent) here like English does.

If I wanted to replace le marin with a pronoun and say "My boss sees him mooring his ship", where would the pronoun go?

It goes before the verb of perception (voit), so you would say: Mon chef le voit amarrer son navire. This is a special rule for perception verbs. Usually, when you have a conjugated verb followed by an infinitive, the pronoun goes right before the infinitive. However, verbs like voir and entendre pull the object pronoun to the very front.

Wait, doesn't chef mean "cook"? Why is it translated as "boss"?

In everyday French, un chef generally means a boss, manager, or leader. The English word "chef" comes from the French phrase chef de cuisine (literally "head of the kitchen"), but in a standard workplace context, mon chef simply means "my boss".

Could I use bateau instead of navire here?