The marine area around the Cinque Terre has been a national park since 1997. We can find interesting rock formations, flora and fauna under the water as well. As I don't dive, I don't have any personal experience, but I do recommend these diving tours. For divers, I also highly recommend a visit to San Fruttuoso and the Christ of the Abyss. Underwater life in the entire Portofino area is amazing. You can find several diving centers in Santa Margherita Ligure. If you just want to do snorkeling in the area, rent a private boat from Portofino and they will also provide snorkeling equipment. My top snorkeling experience was in Paraggi.
If you don't dive, you should at least try snorkeling around the Cinque Terre. Make sure you have your snorkeling equipment or at least swim goggles with you. You can also rent a private boat, and the skipper will take you to the best snorkeling spots.
If you prefer to be above water, why not try sailing, a boat tour from Manarola or Monterosso or a kayak tour along the coastline.
On windy days, Levanto is a very popular surf beach. You can find a surf school here and you can rent surf boards as well.