Well, it just needs to dive underwater. Not deep, just below the water surface.
Then it can still see.
It will even be enough if it sees that one dark spot = ship from below while the sunlight enters the water everywhere else.
So visibility is not really an issue if it dives.
Also how are they supposed to get away after burning their sails when they are on a sailing ship?
Anyone who did campfire knows the smoke is no good for eyes. The last panel focus on Grakos' eyes. So it's most likely the smoke blinds the monster or at least drive it away.
Weak to smoke isn't the plot problem, having to pop out of water first, is.
Since it specifically look for, caught, and "enjoy" one of em without breaking the ship, it could be said that its aim is not to hunt for food
@Markgraf remember the kall? they've been sampling them way before mc comes to that world
Now i only hope that grakos isn't the same as kall which is they kill the girls they play with, guess next chap rino falling to sea with youta saving (touch) her then we can get more uncut action
Is the mc not thinking about his body's situation back at home? He was already crippled after the first fall, I can't imagine what's became of him after the second one.