Definitely don't worry you will get a tan. Just use the sunscreen or you will have a nasty burn and miss much of your vacation! Tan between 10 and noon, at the latest, then slather up again and lay on the beach between 4 and 6 pm.
If you're short on time, here's a quick answer to your question: Hawaii offers a more laid-back, island vibe with pristine beaches and a lower cost of living, while California boasts diverse cities, a thriving job market, and excellent entertainment/culture.
For instance, Hawaii's tropical climate means that it sees more rainfall compared to California. The islands also experience trade winds which help to keep the temperatures pleasant throughout the year. California, on the other hand, has a more varied climate due to its size and diverse geography.
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.