So, it really depends on where you're going. If you are going in Southern California, some of my tips are not necessary.
So, Northern California waters are extremely cold and also a lot more choppy than Southern California. You should definitely wear a 1/4 inch wetsuit and you should have really good strong flippers that will not break.
Scallops can be anywhere from 2 feet below the surface in tidepool like rock settings to 100 feet below the surface. Usually, when I've gone in Northern California, they are around 15-20 feet below. They tend to attach themselves to rocky surfaces and if you see a rock formation with vertical 'slats' on it, they tend to hide in those slats.
Usually a good pull or thwack on the base with your knife will dislodge them, but sometimes they are harder to get off. I suggest you use your knife blade in those cases to pry off carefully.