Can sea turtles retract their heads?

No, sea turtles cannot retract their heads into their shells. Their bodies are well adapted to swimming with generally flatter shells as opposed to the high domed shells of tortoises. Sea turtles have the same muscles as other turtles, which allows them to pull back their heads, but there is simply not enough space in the shell to fully retract the head.

Green sea turtle on a reef in Kenya. Sea turtles have a flatter shell than tortoises and cannot retract their head into their shells. — Sea turtles have a flatter shell than tortoises and have no space to retract their head into their shell. Seen here a green sea turtle.

Tortoises have a domed shell that allows them to retract their head into their shell. Image of a Greek spur-thighed tortoise. — Tortoises have a domed shell that allows them to retract their head into their shell. Seen here a Greek spur-thighed tortoise.

References:

Wyneken J 2001. The Anatomy of Sea Turtles: Part II. U.S. Department of Commerce NOAA Technical Memorandum NMFS-SEFSC-470, 53-112.
Valente ALS, Cuenca R, Zamora M, Parga ML, Lavin S, Alegre F and Marco I 2007. Computed tomography of the vertebral column and coelomic structures in the normal loggerhead sea turtle (Caretta caretta). The Veterinary Journal 174: 362-370.