If every boat is chartered, stop by the Melton Intl store across the highway from the marina.
Explain your situation. They might be able to steer you in the direction of an available charter or private boat owner looking to fish.
I was offered a night shark fishing trip when I mentioned that I should've booked a third charter.
Good people there.
If not - there are half day Ulua trips where you wrestle 100lb GT's right outside the harbor.