While a central conflict of The Cay is a man vs. nature struggle for survival, the book also has an emphasis on racial conflicts. Phillip has been raised to believe that his white skin makes him superior to people of different colors. Throughout the book, Phillip learns that his parents are wrong. He learns that Timothy is absolutely not an inferior human being. Phillip learns that Timothy is an equal. Phillip's blindness is most definitely a literal plot device, but it is also a metaphorical device too. Phillip becomes blind to Timothy's skin color. It simply doesn't matter, and the two characters are able to sit down with each other in mutual respect.
Dr. King's "I Have a Dream" speech displays the same kind of vision for race relations.
"I have a dream that one day on the red hills of Georgia, sons of former slaves and the sons of former slaveowners will be able to sit down together at the table of brotherhood."
The Cay, through the characters of Phillip and Timothy, echoes Dr. King's speech. That is why I think Taylor dedicated it to Dr. King.