Off topic but, my concept of the sailor (or maybe pirate) warlock/monk...
The GOO is the obvious choice for a seaman abandoned to the ocean but the Fiend's Dark Ones Blessing works well as a 'you will be rewarded for sending me souls'. So I had a thought to change things by having a Deep One Patron using the Fiend chassis with the following changes to spells and extra Invocations:
Spells:
Lvl 1: Dissonant Whispers, Command
Lvl 2: Blindness/Deafness, Detect Thoughts
Lvl 3: Lightning Bolt, Stinking Cloud
Lvl 4: Control Water, Evards Black Tentacles
Lvl 5: Hallow, Seeming
Breath of the Deep (requires Deep One Patron)
Your patron grants you Water Breathing at will, but focused only on yourself.
Tendrils of the Deep (requires Deep One Patron, requires 5th level)
Twice a day you can cast Water Whip (as per the Monk ability) without using a spell slot.
What do you reckon?
(I'm thinking Water Breathing works well for when he goes to sea with a pirate crew, scuttles their ship and sends them all to a watery grave, then tramps ashore along the seabed (combined with Devils Sight), hating himself for what he has done.
Also for colour he can sleep under water and comes to prefer sleeping that way. His party would be a bit freaked out, but, you know, Warlock...