If you don't plan to use your free slot for yourself, book your slot on some other day/time so that one of the 18-year-olds can go a second time. The free slots may not be available all at once -they may be available only the morning of the port day. You can venture back there today - the 18 year olds may want to sign up for Flowrider while back there (hopefully the water heater is working).
Biker, who always tries to use the Flowrider on the first as it's likely to be least crowded - though that may not be such an issue on an Alaska sailing in mid-May.