Where do I customize the email that's sent for waitlisted requests?

There isn't a separate email notification for waitlisted requests vs. non-waitlisted requests. You can automate a custom "enrollment form submission confirmation" email to send each time a camper submits the entire enrollment form successfully, regardless of how many/which options the camper selected (and if they're waitlisted or not).

Note that CampSite will automatically append your custom email message with a bulletpointed list of all the options requested by the camper. For example:

To designate your "enrollment form submission confirmation" email, create a new saved camper email, then go to Admin > Campers > Setup Wizard > Step 8: Enrollment form setup > Automatic emails. To learn more, click here.