Experience with surfboard bike rack (or cart)
I agree with Daniel R Hicks- go for a "Bob" trailer style- especially given the distances you're indicating you want to travel.  On the coast, there's always a breeze- and a surfboard turned broadside to the wind will turn your bike into a sail-a-bike and your 150km into what feels like 150,000km.

I'd hit some pawn shops or yard sales and look for one of those aluminum kid-trailers that clips to your rear frame.  You can disassemble the kid-carrying part and have a nice, low frame to strap not only your surfboard but other items you'll need for a trip that long as well.  The extra length of a trailer may not seem ideal to you at first, but I think after your first windy day to the beach, you'll appreciate the low-profile nature.  

Here's the type of trailer I'm talking about: http://compare.ebay.com/like/230660800807?var=lv&amp;ltyp=AllFixedPriceItemTypes&amp;var=sbar