The Rescue of Vincent — Bob Doerr

Fourteen year old Ricky Street discovers a magical coin in a small creek near his parent's Florida home. The coin has his name inscribed on it along with directions that he toss it into the fountain by the Tree of Life. He does so and suddenly finds himself transported to a strange world where he encounters two other teen agers from two separate worlds who had also been transported there by magical coins that they had found. The three soon meet Vincent Wollitzer, whom they save from drowning in quicksand, and learn that if Vincent doesn't reach civilization soon his world will be thrust into a terrible war. The exciting adventures the four experience in their efforts to get Vincent back to civilization include surviving attacks by dragons, sea monsters, and other strange creatures.