But the problem was that a thing was attracting monsters.
Well, attracting is a bit steep. Stirring up, perhaps. (Loosely inspired by some writings on The Lord of the Rings -- Tolkien was not quite clear whether the other troubles the Fellowship faced had any connection to Sauron; I thought the Ring might inspire problems without Sauron's directly desiring it, and without its being a coincidence.)
But if it's to attract trouble, I'm going to have to work a bit more about how it manages to attract trouble in steadily mounting amounts. Part that the ones that come later are more intelligent, but part will not be that they are farther off. Plausible in itself, since people have objections to having dangerous monsters near them. But the heroine goes sailing over the sea -- a pelagic argosy 0:) -- and things could easily come up from the depths if the only objection is distance.
Perhaps the larger they are, the less easily they are stirred. If they are stupid. And the smart ones want to be sure. hmmmmm. . . .