Same issue here on the 3ds version...

In the 1st level of the lighthouse tower (before you unlock going up the stairs to set
things on fire), there are some stairs that go down. I accidentally fell down the stairs,
and there was a bottle down there. There's no way to see it, I just walked over it
blindly.

Either already got that one or missing a different one. Nothing at the bottom of the
stairs.
BTW why aren't there full FAQs or Walkthrough's of the DS versions?

It's your "True Pirate" bottle that's missing. You didn't get to True Pirate first time
around so you never got this freebie. Go to the intro for White Cap Bay and select
"Replay" instead of Freeplay. Then get all the studs you can find (easier if you buy a
stud multiplier from the tavern first).
Once you've got enough studs and the "purse" at the top of the screen shows it's full,
you'll achieve your 10th bottle wherever you are in the module.