What was Dagon's motive?
Let me start by saying I love this film almost as much as I love the story it's based on, but now that I think about it, I'm not entirely sure what Dagon's (or the Deep Ones') motives were in either.
The premise is the same, humans are granted plenty of fish and ancient gold in exchange for worshipping (and occasionally engaging in intercourse with) fish people. But why? At first, I thought perhaps the hybrids were necessary to extend Dagon's rule to the land rather than just the ocean, but then I dismissed it when I remembered that they all seem to eventually return to the water to live among the Deep Ones (in the movie I believe it is only implied, but the story directly states that the hybrid Deep Ones all seem to reach a point in their development where they really can't even be walking around outside and eventually have to return).
I guess "repopulation" sort of makes sense, but only for the film, which only makes mention of Dagon (and not the Deep Ones as a whole). I can see how if he were the last of his kind, he would need to repopulate, but the Shadow over Innsmouth seemed to imply that there were a large multitude of Deep Ones (enough that the fisherman encountered them in another country and a whole slew of them followed him back).