Okay, let me get this right:
1. They are far out on a SAILING ship somwhere in the sea.
2. An enemy appears and that enemy is a SEA beast that is weak to smoke.
3. They set ablaze their sails of their SAILING ship to create smoke to defeat the beast while they are somwhere Oon the SEA as in a LARGE pile of WATER that can put out the fire and prevent smoke from reaching the beast if it just dives.
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Yes, certainly that sea beast will be so simple minded to fight them ON the water instead of diving under water to attack their shiip from below where the smoke cannot reach it and the fire would go out when the ship sinks.
Certainly! The SEA beast that has lived its WHOLE LIFE in the sea will not think of diving back under water to avoid the smoke.
Really, setting the sails ablaze is undoubtedly the perfect solution to their dire situation.
Edit:
This almost reminds me of the "genius" author of that manga where an island from another world merges into Earth through a dimsenion connecting gateway. Then the fishermen from that Island started fishing without permission in the Japanese waters that are regulated to avoid overfishing. And the solution the author of that manga came up with was to increase the amount of fish that are fished by ALL fishermen from Japan and that Island.
I hope the author of this manga will be able to take the curve before reaching the level of that kind of "genius" plot ideas.