Although turtles are mostly aquatic, they are indeed reptiles and not amphibians. Reptiles have a number of traits that distinguish them from amphibians, most of which make them better adapted to life on land. Modern turtles live in water, but their ancestors did not, and they retain these traits.
All reptiles have at least one lung, an organ especially designed for breathing air, whereas amphibians often lack lungs entirely. By contrast, reptiles possess scales that protect their skin from drying out. Amphibians lack scales because they breathe through their skin. Reptiles lay eggs that are protected from the outside world by a membrane called an amnion, and very often a hard shell in addition to this. Turtles, including sea turtles that only come out of the ocean to lay eggs, share all these traits.