Joshua Randall said:
* Instead of setting up Dunwater as a combat mission, set it up as an investigation. So the PCs have found out that the lizardfolk are acquiring weapons. Why? "Just because", or to attack someone? If the latter, whom? You can use Oceanus here to help guide the PCs towards an investigation rather than an assault. Also, tone down the Saltmarsh town council so they are less quick to jump to the conclusion that the lizardfolk are planning an attack.
Or do let them jump to the conclusion, but let the PCs know that they are, indeed, jumping. Have them be typical bureacrats: stubborn, self-important, and narrow-minded. Have them be very snooty towards the PCs (who are, after all, just a rag-tag band of hired mercenaries, whereas the council members are Very Important People).
For example, when the PCs are ushered into the presence of the council, they could be just finishing a debate:
Lowly Council Member: "But if their real target is someplace other than Saltmarsh, sending mercenaries to attack them guarantees that they'll switch their target to us! This mission could actually increase the danger of..."
The Lord Mayor (cutting him off): "Nonsense! We are the only town of real importance in this region - raiding some fishing hamlet wouldn't gain them enough loot to be worth the trouble. Besides, even if they fail, the adventurers should reduce the monsters' numbers to the point where our defenses can handle them."
Baliff: "Ahem. Speaking of which, Your Honor..." (ushers the PCs forward)
Mayor (switching in mid-rant from haughty sneer to used-car-salesman grin) "Ah, my dear sirs! Allow me to assure you that we are all most grateful you have chosen to lend your expertise to the solving of our little problem..."
Also, if there is a ranger or druid in the party, remind the player that her character knows lizardfolk are usually neautral, not evil.