The Merchant Marine spawns in The Legless Arms in the bazaar which is in the far southwest corner at The Intersection of Fod and 1st.
a message in a bottle is the key to get in, its script will send you around some rooms in the world and then eventually lead you to The Legless Arms where you give the bouncer outside the bottle to get in.
I suspect the bottle comes from killing pirate dreadnoughts or super dreadnoughts but unconfirmed. The message in a bottle is also in the loot table of the randomly-generated Safe of Sin.
Also grants the mark of Mark of Landlubbers.
You sense the following: A visiting merchant marine looking for a fight is a 72 year old, level 450 Sahuagin. A visiting merchant marine looking for a fight is immune to: summon charm steal A visiting merchant marine looking for a fight is resistant to: water A visiting merchant marine looking for a fight is vulnerable to: acid disease A visiting merchant marine looking for a fight has a saving against Magic Orbs, Staves, and Wands modifier of 10. A visiting merchant marine looking for a fight has a Saving against Petrification (i.e., Abjure, Kill, etc.) modifier of 10.
High hp, melee damage is about 200 a hit on torso, 80 ar. His bash can skyrocket his damage upwards of 800+ a round. He will raise bar to increase his defense, but can be calmed until this wears off. He will use other knave abilities as well, such as strike down and strike blow. He kicks up dust to blind you, so if you can illuminate that will help you out. Even though the room is solitary, you can summon shaman to help heal you and other NPCs to help damage him. Keep him debuffed and calm often to heal back up, because you will not be able to re-enter if you die.