J.B.
I can't remember exact numbers, but it's not TOO expensive to rent snorkeling gear, however, it's pretty easy to just take your own. If you're going to be there for a week or so, you would probably save $100 bucks or so taking your own -and it doesn't take up much space in the suitcase (especially since you only need some swimsuits, sundresses, flip flops and a few shorts and t-shirts ;-)
Have fun! I'm jealous -and make sure you take the trip out to Molokini Crater -I went diving there, but there were a lot of snorkelers, and it's GORGEOUS -octopus, white tipped reef sharks, turtles, amazing fish -worth it!