Its probably one of the preservatives. Experiment with different beers.
Umm , probably not as SO2 sulphur dioxide is the only preservative allowed in beer .
The level is fairly low , and it might be a beer with a high level causes the hangover .
Try a real ale ? Or as Zimbo says a Reinsheitgebot German beer .
Are you drinking alcohol to ‘hydrate’ after surfing? That cannot possibly work . Chances are you are de – hydrated already and using alcohol ( poison) is not going to help . So your body nicks fluid form any area , around the brain usually , to flush the alcohol ( poison ) out of the system .
So your brain is not suspended in a fluid but banging into your skull , hence the headache .
Try drinking 2:1 water : alcohol , or neck 2ltr of High5 Zero before cracking open a cold one , then another later on.
NB .- All of the above might not be entirely true , but as a brewer and a windsurfer i have some experience in the area .