No matter when you go to Koh Samui, it will be sunny and bright throughout. So, sunscreen is a must for your Koh Samui tour. Take a good one with a higher SPF value, so that you can stay protected throughout your holiday. Also, take a hat and sunglasses with you, to stay extra covered.
Wondering what if your skin still gets burnt? Well, this is why you will have to take a good lotion also with you.
Note- If you are a traveller who doesn't normally pack a lot of liquids when travelling to a new destination, and takes whatever the creams and lotions from where you step into, keep in mind that you can hardly do this in Koh Samui. Sun protection creams and lotions in Koh Samui can be very expensive, and sometimes their SPF values also are less. So, the wise decision is to take them with you when you go.