This description occurs when Sanger Rainsford, the protagonist, finds himself swimming towards the mysterious island after falling overboard.
Here's the exact passage:
> "The sea was cold, and Rainsford's teeth chattered. He was swimming strongly, but the island was still a mile away; he had been in the water almost an hour. A dark, shadowy forest loomed at the edge of the beach. Rainsford's heart pounded."
The vivid imagery of the dark forest against the bloodwarm ocean creates an unsettling and ominous atmosphere, foreshadowing the danger and suspense that await Rainsford on the island.