Mr Shatner discovered the body of his wife, Nerine, a 40-year-old former model, when he returned home at 10.15pm on Monday. He pulled her out of the water and tried to revive her while waiting for an ambulance to arrive. She was pronounced dead half an hour later.
"My beautiful wife is dead. She meant everything to me," a distraught Mr Shatner, 68, told reporters outside his home. "Her laughter, her tears and her joy will remain with me for the rest of my life."
Authorities said an autopsy was under way but they did not think any foul play was involved. The former Nerine Kidd was Mr Shatner's third wife. They married in 1997.
Two months ago, fans of the series mourned the loss of DeForest Kelley, the actor who played Dr McCoy. He was 79.