You should be prepared for the sun because the sun is really shining. You can easily get burned if you don't wear sunscreen. The sunscreen is your vital thing when you are visiting Koh Samui. And remember to wear sunscreen also when you are swimming!
Be careful with the sunshine. You always have to have sunblock with you. Don't stay too much under the sun. Buy sunblock at home. I am not sure the thai sunblock are strong enough for Europian people.
The sun here is very strong even if it is raining or cloudy...........use strong suncream and be senseble your holiday could be ruined