No. You can do what's called "discover scuba." They'll take you to a pool, usually, and teach you how to clear some water from your mask, as well as recover a regulator and clear a regulator if it falls out of your mouth. (Still has never happened to me.) Then they can take you on a trip to go diving in the ocean.
I've never been to St. Lucia, but if a dive shop tells you you need to be certified to go diving, tell them to go fuck themselves and you'll take your business elsewhere to a place that offers PADI's discover scuba.
You will, however, be limited to the sites you can dive. You'll still have a good time, though.