Final answer:
The irony in Rainsford's thoughts lies in his misconception of safety and overlooking the true danger he faces.
Step-by-step explanation:
The irony in Rainsford's thoughts "all he knew was that he was safe from his enemy, the sea..." stems from the fact that he is actually in great danger. Rainsford is being hunted by General Zaroff on Ship-Trap Island, so the sea should not be his enemy. However, he mistakenly believes that once he reaches the shore, he will be safe from Zaroff's pursuit. The irony lies in the fact that Rainsford is overlooking the true danger he faces, which is the cunning and ruthless General Zaroff.