I think it makes sense, all things considered. The fishermen were drawn there by that mind flayer. But the devourers were on the ship already. You can see several of them running around while you're heading for the helm, and I suspect that these are those. I don't know though, but it seems logical given the circumstances. Now maybe the circumstances may seem illogical, but it's the story we have, and the plotline they're working from.
Again, story wise, sure, there could be 100s of heavily nerfed intellect devourers roaming around, but I stand by my point: lore wise it makes no sense to come across more than 1, 2 would be pushing it, and not as early as lvl 1. Like they could've replaced these intellect devourers with mind controlled humans or whatever, and it would make much more sense than what we have now. I know, killing intellect devourers and seeing walking brains is much cooler than whatever I suggest... Also, don't you find it weird that Shadowheart somehow has 4 dead brains at her feet and 5th very close by but if you send her against those in the ship, she'll most likely go down? Ah, one of these little mysteries that will most likely never be explained.
