Frederick Weller, who portrays Jay Bennett (the antagonist), is a cousin of Peter Weller, the director, and who also guest starred in the season 5 premiere "Mr. Monk and the Actor (2006)."
The marina where Monk goes to investigate the fender bender case is the location of the Psych Office in the TV show Psych (2006) which was also a USA Network show and set in the same universe as Monk.
Monk states that he has never been swimming and only learned via correspondence course, but in "Mr. Monk Takes a Vacation (2002)" (S1:E9), Sharona teaches Monk to swim.
In the restaurant scene, according to the credits, the character played by Mark Teich is the "Manager" but in the dialog he has a name, Hal. And the hostess's name, Courtney (played by Odette Annable and credited as 'Odette Yustman'), is given only in the credits but never used on screen.