D&D 5E Help with a homebrew warlock problem

[Lanliss]

So, those who looked at my warlock thread know that I am giving warlocks free bonuses at 3rd level, themed around their specific patron, and depending on their chosen boon. I am now playing a Bladelock of Davy Jones, and plan to ask my DM about letting me get a similar ability for him. Knowing Davy Jones is an Undying Patron, themed around Pirates, what would be a good Bladelock ability based on him? I seem to have hit a bit of a block on this. Keep in mind, I am trying for the abilities to be generally balanced, and not too much stronger than a base Warlock, just a small flavorful bonus.

 

[THEMNGMNT]

Underwater breathing, no attack penalties underwater, swim speed.

 

[Lanliss]

Underwater breathing, no attack penalties underwater, swim speed.

Ooh, nice one. I will need to bring that up to him.

Edit: Comparing it to others I have, I think it might be a little bit strong. I think I will go without the Water breathing. I will make that an available Invocation instead. Do not know if my DM will go for it, but I will be fine with it if my players want to play such a warlock.