Tony-
It is VERY legal with exceptions. Please use this link to the Hawai'i DAR (Divsion of Aquatic Resources) for specific information.
http://www.hawaii.gov/dlnr/dar/index.html
If you plan on using a fine mesh net to collect aquarium fish you will need a permit:
"10) Aquarium Permit - required for any person to use fine mesh net for collecting aquatic life for an aquarium. Report required if commercial. HRS §188-31."
I have done this many (3+) times and have never encountered any problems. The most trouble I had was at the airport (in Honolulu) when the Aqriculture inspectors thought a cowrie (via x-ray) was an orange and they searched all my bags - they were only looking for agricultural products and could have cared less about my aquatic life. Remember, Hawai'i IS a US state and there are no customs to go through when you get back to California. Follow the regulations on the DAR website and everything will be perfectly legal.
I have always brought my catch (usually hermits, brittle stars, cukes and snails) on the plane with me without any problems. I would suggest getting some Kordon breathable bags from Monolith Marine Monsters so that oxygen is not an issue (brought four Ventralis Anthias back from the Cook Islands - they were caught by a collector there - using this method - total bag time 26 hrs).
Remember though

!!!) all coral (soft and hard) collection in Hawai'i is absolutely forbidden (!!!) This also includes live rock.
Also, please make sure that where you are collecting from is not a marine conservation area (areas are indicated in the website).
Have fun!! I still have the brittle stars and cukes I collected there four years ago!
sedgro