I was last in Belize ten years ago on a back-packing trip, so things may have changed but:
One or half-day guided trips were readily available on Ambergris Cay, which is the main tourist area. I didn't partake because I was travelling on the cheap.
I had good non-guided success for bonefish off the docks (first light before the people show up) and beaches at Cay Caulker, which was the main backpacking destination back then. At that time you could rent a sea kayak hourly or daily to access some of the more remote parts of the Cay.
Make sure you bring your own flies and gear because at least back then, there was very little for sale.