This is alot more common question than you may think.
Snorkeling from Shore or from a boat in Maui it is extremely unlikely you will see a shark or get bit etc.
You are far more likely to be in a car accident while driving on Honoapiilani Highway.
(Tiger Shark Swimming at Black Rock Ka'anapali)
Here are some Stats
(In the Hawaiian Islands, since 1828, there have been 116 confirmed shark attacks and only 8 confirmed shark attack fatalities. For Hawaii, that averages out to a shark attack every 1.59 years, and a fatality approximately every 23 years.)
Now all of these shark incidents usually have a few things in common.
1.Murky Water/Turbulent water
2.Fishing/Spearfishing in the area
3.Recent Whale death
If you would like to see an updated list of ACTUAL Attacks/Incidents here in All of Hawaii/Maui check out this website