If you have a reservation on Tropic to go back to international in Belize City 15 minutes is adequate. However, if you are coming down from the north you MUST allow for as much as an hours delay at the pull ferry. At the international they insist on 2 hours and people have been refused check in if they cut it less. Better be safe than sorry as you can't just 'hop on' the next flight out.
Check in and go through security and browse the gift shops inside - I found the best buys in there.
If you want to eat, check in but do not pass through security and walk (no elevator) upstairs to the restaurant upstairs. Decent food, good service and no gouging on the prices.