The phrase "night snorkel" refers to the activity of snorkeling, which is exploring underwater with a special mask and breathing tube, but doing it in the darkness of night. Full definition
We had opportunities to night snorkel with Manta Rays, day snorkel along the reefs with colorful fish and sea turtles, Kayak along the coastline, and visit historic places along the way, learning about Hawaiian lore and traditions. (uncruise.com)