"We had a couple scenes where he's swimming. Let me tell you, I have not enjoyed a swimming actor for a long time," star Sasha Alexander, who plays Isles, told TVGuide.com.
Gosselaar guests as Garret Fairfield, one of three brothers from a wealthy family in Boston. When Garret's brother is found dead, he and Isles reconnect.
Not only does his character's swimming background play a part in the episode, but Fairfield "shines a light on [Isles'] background and where she came from, [which causes] tension between her and Angie [Harmon]'s character," Gosselaar says.
Alexander offers a warning to Saved By the Bell fans: "He does some real shady stuff here. Zack Morris, how could you?!"
The episode airs Monday at 10/9c on TNT.
Additional reporting by Joyce Eng