In general, Polly supports tanker formation. His "Instant Regret" and "Tough Nut to Crack" are the proof that he needs tanker(s) in the formation. So if you have tanker formation with multiple tankers, Polly could increase formation's DPS
Be careful with "Got a Cracker!" ability, if you don't want enemies to come to your formation too early, don't buy this ability
Polly is very useful for the mission "The Haunted Ship" in Captain Ghostbeard's Greed campaign. His "Got a Cracker" ability causes the spawned ship to arrive more quickly and limit the gold reduction time.