I have never taken for granted that I will be able to find an excursion guide waiting at the peir to take people who are not already signed up for a tour. Also, Belize is a relatively new stop and there may not be a flood of local operators with excess capacity. And would you trust your safety and that of your companions to someone you had not done any research on?
I would either book the ship tour (they at least conduct a screening process for vendors) or check on the internet and see if you can find someone who operates outside the ship tours and make reservations with them.