It's sort of like how people say, "What's up, dawg?" and the reply is "Chillin'" or something like that. You can use it even if your ship isn't docked or anchored.
The coolest part about all of this is that you don't even have to be a sailor to use this word. Everyone can use it! Ninjas, Space Aliens, Illegal Aliens, Legal Aliens, Ninja Assassin's, Homies, Bros and even Super Mario!
Jason: Hey Erin, how's you day going?
Erin: It's docken. How about yours, you sick twisted sailor?
Jason: It's pretty dang freezin.
Erin: Wow, you've just used a term that's interchangeable with chill!
Jason: I believe I have. Since I am such a docken man, we shall proceed to sail, dancing and singing to the song I'm on a boat!
Erin: Oh yeah!
Dokken is the sailor version of chill, except you don't have to be a sailor to use it.