The Movie Dagon (2001) is adapted from The Shadow Over Inssmouth by H.P. Lovecraft.
There are quite a few differences from the original Novella and the movie. At first, in the Novella, we can perceive the main character slowly descent into madness by the use of complex descriptions. This is very difficult to reproduce in movie format. That because the type of horror in Lovecraft's stories is interpersonal, most of the times, even when there are monsters and other beings beyond human comprehension. Which brings us to the next point. Lovecraft's creatures are usually so alien that the human brain can't process them correctly, which contributes to the sort of madness that the author describes, and it's also quite difficult to reproduce in video format.
In the end, although it's a great homage to the father of the Cosmic Horror, it was not with this movie that they managed to translate his work to a different medium yet.