I am a scuba instructor and recommend doing your resort dive with a reputable dive shop. Of course, even better would be to get certified before you go (if there is time). I am not that familiar with the shops in Placencia, I'm sure there are some good ones, but know definitely that there are many good ones in San Pedro. I would be glad to recommend some to you.
The diving in Placencia is excellent, though the boat ride to the reef is much farther. Diving in San Pedro is good, the reef is much closer. I think for the resort dive you will probably be taken to Hol Can off of Ambergris. It is a protected area and filled with fish life. Be sure you snorkel at Shark/Ray Alley, it is fabulous. I live outside of Pittsburgh now but did live in Belize for 5 years. For a list of dive shops I know of and trust, feel free to email me at: