If you book a party of 3 for parasailing on CC, you will pay for three. Regardless of how many flights you have, solo or otherwise.
Board the boat with dollars. ($20-70) Talk to the crew before or just after debark regarding your concerns. Assess other pax groups, that may be compatible with your teen. Often there are other three party groups aboard thinking the same thing. Often with a parent that doesn't want to sail, as a parent must be present with minors IRRC. The crew know this dilemma prior, or shortly after debark.
They do a wonderful job of assigning compatible teens together if necessary with approval.
They will never put an unrelated minor male and female pax together.
Whatever scenario you have thought of, they have encountered more. Trust me.
The crew will talk with everyone aboard about the best way to handle this. Similar to a flight attendant seating families aboard a flight. lol.
Winds play a huge part of the final configuration. It's all about weight, winds, seas, and whatknot. Min 90lbs, max 375lbs combined. In high winds they want a pair of pax to weigh 200 - 400lbs, IME.
They will work out a solution acceptable to all aboard prior to the first flight.
I know $$ may assist your priorities.
The crews are great.
Don't forget to bring your cameras, not expensive DSLR ones. It can be a rough ride out to sea. Everything could get wet. Bring a cheap camera, make friends aboard if necessary to take pics of you with your camera. You will obviously reciprocate.
It can be a very calm boat ride too.
Take Bonine the night before and in the morning. Give kids half dose. Did I say it may be rough?
It's an insurance policy. Just in case. If you plan, and take meds for rough seas, you will have none. IMO . Ever heard of Murphy's Law? JME
Book this excursion if possible, it's one of the first to be booked up fwiw. Very popular.
Book the earliest appt if possible. Early usually is the calmest weather.
All appts cancelled when winds hit 25 -30 mph. Or less depending on island time or whatnot.
Don't tell the kids about this excursion without explaining the 30% chance of a no go due to weather/docking. JMO.
Once docked at CC and weather good, they will learn soon enough about parasailing.
We have tried 6 times and sailed thrice. Failed to dock twice fwiw.
BTW did I say this excursion is awesome regardless if you get soaked outbound or inbound.
Book it if possible IMO and hope for the best and go with the flow.
And yes, you, as the adult may end up flying with both your teens. Although often the parent waives off to the other teen. So one teen may go twice with the original related teen, when winds and conditions allow.
Trust me, don't fret. The crews rock.
What you should be worried about, is getting a booking for this excursion.
https://disneycruise.disney.go.com/port-adventures/castaway-cay-parasailing/?sourcecode=16102&CMP=KNC-FY16_PS_DCL_TRA_DOM_Branded|G|4163700.CL.AM.01.03&keyword_id=FRS1UPcy_dc|+castaway +cay|70472088243|b|1540tw413989
HTH