Hawaii - Kaanapali hotel day rate or lockers?

Will have a day in Maui and want to snorkel Black Rock. Anyone know of day rates or place to check belongings?

Don't know of anyplace with day rates in your situation.
The places with day rates are given as a courtesy(fee) to guests who have stayed there and have a overnight flight on their last night.
Your other problem may be that if you decide to rent a hotel room for a day, checkin time is usually 3-4 pm.

Why not just make a day trip to the Sheraton Black Rock? All the beaches are public on Maui. Worst case scenario is that they'll charge you to park, though most hotels offer a few public parking spots for locals. Most resorts have showers right off the beach so that you can rinse off before heading back to your hotel.

Actually, I'm there on a cruise ship. Problem is leaving things on beach while snorkeling.

Perhaps renting a car, using it as my locker and just parking is the best plan. Stayed on Maui many times, so a day at Black Rock and Lunch, with an afternoon sail sounds better than a Molokini Snorkel at this point.

If you decide to rent a car and use it as a storage locker, i suggest Valet parking it at the Sheraton Hotel proper.
Not at the public lots.
Make sure you tell the valets to lock the car when they park it and also keep an eye on the car.
A generous tip helps in the process.

Ah, I see. Whenever I go snorkeling or hiking, I use Aquapac's to store my keys, money, credit cards, etc. They also make them for cameras. I wouldn't worry about towels and stuff.

http://store.aquapac.net/explore-product-range.html