This question comes up a LOT - and understandably so. Folks really want to know the answer to this question when planning their trip. After all, the main reason most folks go to Hawaii is to swim/snorkel/play in the water.

Well, the truth is, you may be surprised to find out that compared to some Caribbean locations (or even the U.S. Gulf Coast) Hawaiian water temps are not as warm. Florida, Texas, etc... all have much warmer water in the summertime than Hawaii does, and many popular Caribbean locations such as the U.S. Virgin Islands have winter water temps that are about the same as the summer water temps off Hawaii. This of course goes a long way in explaining why the Caribbean and Gulf Coast get so many Hurricanes while in Hawaii they are relatively rare. On the other hand, while those Caribbean locations DO have warmer winter water temps than Hawaii, the winter waters of the U.S. Gulf Coast and Florida are generally quite a bit cooler than the winter water temps off Hawaii. So overall, compared to those warm water areas of the U.S. Gulf and Florida, Hawaiian waters are warmer in the winter and cooler in the summer.

As a general rule, Hawaiian waters average 77 degrees in the wintertime to 82 degrees in the summertime.

Now, compared to the U.S. West Coast, Hawaiian waters are definitely much warmer (10-20 degrees warmer) than the waters off Southern California. No comparison really for the U.S. West Coast.

And for the average high and low temperature of the air, plus average rainfall, as well as the temperature of the water, for each individual month of the year on Maui, see the chart on this page:
Maui Weather

It's the National Oceanic and Atmospheric Administration site showing montly average (and in some cases actual current) water temps all along the the U.S. coast, from Alaska to Maine and out to the various islands of Hawaii.

An extremely useful and interesting site for those planning a vacation to Hawaii - or a beach vacation to any U.S. coastal area actually.