You'll find the best underwater sights off Pu'u Keka'a, also known as Black Rock, the lava promontory that protects the beach in front of the Sheraton. First-time snorkelers will be happy with the coral and fish at the protected southern side of Pu‘u Keka‘a, but the highlight is the horseshoe cove cut into the tip of the rock, where there's more pristine coral, abundant tropical fish and the occasional turtle.
There's often a current to contend with off the point, which can make getting to the cove a little risky, but when it's calm you can swim right around into the horseshoe. Pu'u Keka'a is also a popular shore-dive spot. If you want to see what the horseshoe cove looks like, take the short footpath to the top of the rock, where you can peer right down into it.