The sea turtle that helps Marlin in Finding Nemo is Crush.
Marlin meets Crush after he and Dory are swept into the East Australian Current. Crush is a relaxed, friendly green sea turtle who helps the anxious clownfish understand that raising Nemo requires trust as well as protection. Their conversation gives Marlin emotional guidance during his search for his son.
Crush is traveling with his young son, Squirt, and the two are part of a large group of turtles riding the current toward Australia. The encounter also provides one of the film's most memorable comic sequences, with the turtles using the fast-moving current like an underwater highway.
The character's voice has an unusual Pixar connection: director Andrew Stanton originally performed Crush in story-reel recordings and retained the role after test audiences responded positively. Crush is sometimes confused with Squirt, his son, or Mr. Ray, Nemo's spotted eagle-ray teacher. Neither is the named turtle who befriends Marlin; that character is Crush.