Ha, so it turns out that Key likes to slip characters named "Saito" into their works for laughs. There's Jet Saito in Clannad, Bush Saito in Kanon, Mask Z Saito in Little Busters! and Fish Saito in Angel Beats! ...apparently there's a Hop Saito out there as well but I couldn't find him/her.
The thing is I'm not sure if the jet ski dude IS Jet Saito... When he appears, his name is displayed as 「ジェット男」 (romaji: Jetto otoko, literally "Jet Male"/"Jet Man"). After he leaves, Sunohara talks about a junior SCHOOLMATE called 「ジェット斉藤」 (romaji: Jetto Saitō), kinda like a guy named Saito who has the title "Jet" (like "Rocket Dan", I suppose).
Saito is a reasonably common surname, so it has no translation anyway. For the label, I'm inclined to use "Jet ski man" instead of "Water jet man" as "jet ski" refers to the whole vehicle but "water jet" refers to the propulsion system only.
Anyway, our Saito isn't tied to Tomoyo's route only; the key to summoning him is to ditch Sunohara every chance you get (one possible route out of two is in
this walkthrough).