You can get back on the same day if you take first ferry, which leaves early..if you take late ferry you will have to stay overnight.
Getting back to Fajardo the same day has nothing to do with which ferry you take TO Culebra.
There is a 5pm ferry from Culebra to Fajardo. If you miss that one, you'll be stuck on Culebra for the night.
Getting tickets mid-week in September should be relatively easy. Though, I would still get to the terminal in Fajardo no later than 7am (for the 9am ferry). If it turns out that you don't make that ferry, there is a 9:30am ferry to Vieques ... so have a day at Sun Bay on Vieques "in your pocket" as a back-up plan.
You cannot buy tickets in advance.
Thank you. I love the back up plan idea!!
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