Thought some of it over and decided to toss up some proposed changes.
If you shoot an enemy champion in the back, the spell will slow him by X% (probably something low)
Gangplank tries his very best to be a chaser with his high movement speed and the ability to remove CC, but he just doesn't quite make the cut. Consider that most all melee heroes have an ability to close the distance to an enemy hero AND slow/stun/silence them. Gangplank has none of these. Teemo has a similar skirmisher playstyle and he has more CC and doesn't have to bother with melee range.
Either of the following.
1) Reduce Cooldown
2) Grant immunity to movement impairing effects for a very short duration. (mini-highlander)
Remove Scurvy is a great move, but does next to nill in team battles. Even if there are just two heroes around him, he will easily be CCed again before he can get away. Consider the CD of Remove Scurvy to the thousands of spammable nukes. Consider Master Yi who has Alpha Strike as an initiator, much higher damage, higher movement speed, and a much more useful anti-CC (highlander). This wouldn't be a big change, just something to put him in line with other heroes.
Change it to just an activatable (no shooting a creep)
-Toss in an animation that will shoot the nearest creep thats low on hp and have the Pirate laugh insanely if there are friendly creeps nearby.
A lot of team battles, the only place worth having this, happen away from friendly creeps. Even in a team battle nearby friendly creeps, it can be really difficult and time consuming to have to click the ability and find a creep with 5 enemy Champions running around. This severly limits this ability. Changing it wouldn't make the ability any more powerful, just give it the use that it was intended to have.
-A change is apparently in the works, so no real need to discuss it.
//My two cents//