Are any of them lawful? He could have them go shakedown one of the other boats for their protection money rent and if they still refuse have them cast off the boat from the mooring with passengers still on it. Paying rent is a fairly lawful expectation and so would a resulting eviction, even if it clearly is morally (read good/evil) problematic in this case.
He could also tell the party they have to make a trade for some of his men that are in Kroft's jail, so they now have to go back and convince her to let the thug(s) out. You can liven it up even away from the boat by having Kroft tell them what the guy's crimes were and why Devargo's men are dangerous.
I've also had an escaped member from All the World's Meat (if you had one) appear as one of the guys trying to be recruited in Devargo's office. Can at least create tension in the room.
And finally, you can offer the party some gambling games on the gambling boat if they do wind up coming short on money, or otherwise want to try and just make even more coin.