Still, I'd like to see – especially if you expand this into a full-fledged 'Return to Ixalan' set – some green and white and Non-Pirate stuff.
You've been improving a bit – the second post's cards felt more cohesive and restrained than the first. See the spoiler below for cards that need to be looked at.
Oh and there's one more thing – I really suggest you add some more creature damage, destruction & bounce [return to hand] cards. It's what Grixis [Blue, Black and Red] is good at, and it's especially important in a set with so many creatures.
Fathom Fleet Scavenger seems just a little bit too powerful for a common. I'd suggest making it cost 1 more, making it loot one less card, or making it uncommon.
Dire Fleet Arsonist – it seems like this design is rather cumbersome with the amount of tracking and memory issues it entails. Imagine this in a long game of Commander.
Rapid Pillaging seems much too powerful if it sticks around, even at that cost. This is especially concerning if you imagine a limited [Draft or Sealed] environment, where enchantment removal effects would be limited.
-I would suggest simplifying it and making it empty at end of turn, which would also reinforce the 'rapid' part.
-Rapid Pillaging 1RR Enchantment [Rare] Whenever a creature you control attacks, put a Plunder counter on Rapid Pillaging. At the end of your turn, remove all Plunder counters from Rapid Pillaging, then put that many Treasure artifact tokens into play.
High and Dry isn't bad, but it still seems a tiny bit too good. Maybe just a 1/1 token instead of a 2/2?
Is Johnavier Voice of the Free's stealing ability supposed to return them to your opponent at the end of the turn? If it's not supposed to, I don't know why the second line, with the not untapping part, is there.
Captain Sarina's effect is too powerful. Not only is she a good 'lord' for the pirates, but her effect lets you cheat out big spells repeatedly way too easily.
Gwyn is a bit too powerful – Indestructible on top of gaining plenty of life? But it wouldn't take much to make this card fairer – maybe just a single X, instead of double X.
Grayson is definitely too powerful. Specifically, he is much too cheap for that effect. If you wanted to keep those effects as they are, he'd have to be 3UBR.
Swab is a bit too powerful – each effect would be fine at uncommon on their own card. If you want to keep these effects together, move it to rare and make it cost more.