To set up swimming, you should set it up as a club.
This will allow guardians to sign their children up through the Parent Portal and Arbor App. It will also ensure that the event will only show in staff calendars for the time they are actually supervising the swimming.
Do not set swimming up as a trip. If you set it up as a trip, staff assigned will have their calendar blocked out for the entire duration the trip is running, which will cause issues when booking Guardian Consultation meetings with these staff members.