Turtle is a giant old creature from The Legend of Zelda: Majora's Mask. Link wakes him up in his Zora form with a wicked bass solo. In return, he offers Link transportation and wisdom.
And he's really your only way to the Great Bay Temple, so thank Mr. Turtle for helping you finish your quest.
I also like it's cameo in the Great Bay stage in Super Smash Bros. Melee on Gamecube.