To be fair, he could always claim he was being in character later. Part of the job, all that jazz. Being cautious might be overall a good idea, but it's not an iron rule when you're pretending to be an adventurer for money.

Squalo listens, but more so than the moral of the story or the fact that there's supposedly a cursed treasure, he seems to be appalled by Barbossa's betrayal. First mate sounds a lot like right hand man for some reason. Someone you're ought to trust unconditionally --

"Wait, stop right there. Your first mate left you to die? What the fuck?"