Know who needs a pirate skin?
CAPTAIN WHITEBEARD ZILEAN!!!
Splash art could be of him as this imposing, towering figure way larger than the rest of his crew, staring down at them oppressively as he holds a fuse-lit pirate grenade in one hand with several more strapped to his belt, perhaps wearing a wicked half-smile as he sees his crew pushing themselves to avoid his punishment. Otherwise I leave the actual artistic design to an actual artist.
Passive: Every time he channels it, he coerces his 'crewmember' into working harder... or else! Some lines could include: "Slackers get the plank!" "Harder! Faster! Better!" "Don't make me light a fuse under ye!" "Pick it up or you're next!" "Don't make my list, whelp!" "Rest when you're dead!" "Not good enough!" "Idle hands get cut off!"
Q: He throws fuse-lit pirate grenades. Basically just hollow iron spheres with powder and a wooden fuse plug. You see them in Pirates of the Caribbean. Anyway yeah I imagine their fuse burns hotter the closer to detonation with the number still staying above as normal.
W: Instead of a huge clock/pie chart appearing above his head, you see a large animation of one of his grenades being lit, accompanied by the sound of a stricken match.
E: A whiplash sound effect plays from the buffed character, who will have red 'strain' pulse around them when the buff is granted. If Whitebeard uses it on himself, you instead hear a flintlock being fired into the air from him, keeping more standard 'movespeed' animations on him. Or you can use that for all buffed targets instead of the whiplash. Just two separate ideas.
R: The buffed character has a bubble of water enveloping them with the three spheres being replaced with a skull, crossbones and a blank black flag. If it procs, the crossbones then the skull merge onto the flag, once again creating a full jolly roger, with an audio cue playing over the resurrecting character. Some ideas for the audio cue include: "I release you from Davey Jones' Locker!" (ominously) "A pirate's life!.. For meee..." (If procced on himself) "Raise your flag. You're not done yet." "The deal is struck."
Autoattack: If you're willing to have a new animation altogether (make it an ultimate skin, I don't care!) he could use a flintlock. Otherwise he could throw pirate coins that make coin noises.