If you're really not sure about dates, you can get a private dating scan from about £40-80 depending on your area. This will also rule out ectopic pregnancy and let you know if everything is progressing as it should. And you get a picture to take home. They often have availability at short notice, so you could get one before you go on hols.
Regarding surfing, I'd probably do it, if I really wanted to, and was in the first trimester as the tiny tiny foetus is still well in the pelvis so a typical injury (hit with a board, broken leg?! I'm a skier your boards are well out of my area of expertise) are unlikely to affect the bean and a catastrophic injury is unlikely.
Regarding being cold - it's not the flip side of being hot. The reason hot tubs aren't recommended is that there's a link between raised body temp due to fever and neural tube defects especially in the first trimester. Being a bit cold (unless you're actually freezing) won't affect your core temp in the same way.
I really rated 'Expecting Better' as it explains the science behind recommendations, e.g. caffeine, not cleaning the cat's litter box, avoiding some fish...so lets you make up your own mind. I also liked the Mumsnet Pregnancy / Babies books as a kind of condensed version of the irreverent yet practical advice you get on here.
Finally, it sounds like you might be a June due date? Whatever it is, have a look in the antenatal clubs and you'll find loads of people in the exact same boat. I'm five weeks myself as it happens...
Best of luck!