Practice is a must if you've never been before. Before my wife and I went on our first cruise ~5 years ago, we went to a local dive shop (Aquatic Center of Rochester) that actually has an indoor pool, and we took a snorkeling class. We had never been snorkeling before, and it was a great way for us to 1) try it out to see if it was fun (it was!) and 2) gain some confidence in a safe, controlled environment. Then, when we went out on the guided tours, we didn't have to waste time figuring out how things worked since we had some experience. Taking a breath while your face is in the water feels quite unnatural until you get used to it.
I also recommend bringing your own snorkels. Some places give you a cheapo snorkel that you get to keep. Some of the boats just dunk their snorkels in a big tub of gross looking water. It probably had bleach in it, but still... Besides, the provided snorkels are usually the basic ones with no drain on them (I've got no idea what the proper name of it is). The ones we bought have a place to pinch it near the mouthpiece that lets you exhale and blow out any water/spit that might have gotten into it, which we found to be a nice feature.
Have fun!!