What literary device does conell achieve by the opening argument between Rainsford and Whitney aboard the yacht?
There are several. Foreshadowing is the use of hinting at futuristic events. Saying that the island is "a mystery" and calling it "Ship-Trap Island" and describing "the crew's nerves" as "jumpy" all foreshadow a sense of foreboding on this island.
Similes are comparing two unlike things using the words 'like' or 'as'. "It's like moist black velvet" is a simile when Rainsford compares the night to velvet; "The sea was as flat as a plate-glass window" says Whitney describing a "sudden chill" as they passed the island, yet there was no wind by the evidence of a flat sea; the sea in this sense is compared to a window.
An understatement is when writers say one thing and they mean just the opposite; it is a deliberate "underplaying or undervaluing of a thing to create emphasis" (Roberts & Jacobs 2008). When Rainsford says that he can't see the nearing island, Whitney responds, "You've good eyes," with a light laugh. Rainsford has more than "good eyes," he has excellent eyes because Whitney describes his (Rainsford's) ability to see moose four hundred yards away blending with their environment and still being able to kill them directly. Rainsford's poor eyesight metaphorically is his inability to see his own danger and vulnerability to this island.
Personification is giving inanimate objects humanlike qualities. The "night that was palpable as it pressed its thick warm blackness in upon the yacht" suggests that the night, an inanimate object, can "press"--an activity that humans do (press buttons, press against someone, etc).
A hyperbole--an exaggeration--is seen when Whitney describes Captain Nielsen as a person "who'd go up to the devil himself and ask him for a light." Captain Nielsen is mean and fearless of many things, except the island. Whitney is proving that Nielsen's inability to fear the devil, but yet fear this island also supports the assertion that there is something terribly wrong with this place (island).
The easiest literary device to see there is foreshadowing. They don't just introduce the island, or where they are, but the fact that the island is a mystery, and that it traps ships in some fashion.