F4.0 should be fast enough. Afterall the light conditions while surfing is usually pretty good (i guess). Just pick ISO 400-800 and yould should be able to get shutterspeeds around 1/1000 sec or faster.
f4 should be plenty!
as far as night surfing- i doubt you'll even get great shots unless a flood light were present...
even a 1.8 super duper zoom will have a hard time focusing in that amount of light.
my 70-300 4-5.6 captures quick action moments in the desert with no issues.
as far as working in the water, you might want to invest in a weather sealed armor and a jetski along with someone who'll drive with you to the action.
personally, i think a lot of sport shots look extremely bland when shot from far away. you've got to get in on the action and a combination of a 70-300 or 70-200 2.8f lens along with a jet ski would be perfect as it allows you to get a better perspective on the surfer.
you're actually local- give me a call if you want some more advice or questions. pm'ed you phone number.