on the island with preffesor hammerhead
When you have sedated the shark, you can swim out to the island in Booga Bay to bring him back.
Shark boy was born on July 21 2007 and born next Booga Bay on Shark Tooth Island.
Swim right to the island to rescue the missing boy. Lead him back to the beach.
Shark tooth island is when you have to to save people from a giant shark
You return the boy to his mother and the Professor gives you the Island Medallion.
The mission on Shark Tooth Island is to rescue the boy who is stranded in an island in Booga Bay (far right of the island) by the giant shark Booga. The solution is to gather the ingredients for a sleeping potion, to be mixed by the island's medicine man. (see related question)
Unless it is for sale at the Store, you would have to get one with the costumizer. (Did not try Shark Boy on Shark Tooth Island.)
swim across the water to meet an old guy and the little boy
You don't, it's just a decoration. You can also get the whole "shark outfit" by copying it from Shark Boy in the museum.
There are no mermaids in Shark Tooth island, you do go into the ocean
At the completion of Shark Tooth Island, you rescue both Professor Hammerhead and the missing boy, both trapped by the menacing giant shark. The "calming potion" (knockout drug) created by the medicine man puts the shark to sleep and you can swim out to the island. (see related question)
Shark Tooth Island was the second island on Poptropica and premiered in September, 2007.