The water will reflect the rays of the sun, so if you are in or near the water, you will more easily get burned. On our last trip to Koh Samui, we took the very early morning flight, because it was several thousands of Baht cheaper than any of the later flights. We arrived in the hotel around 8 am and we knew that we couldn't check in that early, so after eating breakfast we decided to spend the next 4 hours resting on the beach in the shade of some trees. Since our sun protection was still in our suitcases, which we deposited in the reception, we didn't go out in the sun at all, but still my sensitive farang-skin got very red and painful, so I had to avoid the sun for the next few days. We will soon be travelling to Koh Samui on the very early morning flight again, but this time I will make sure to grab the sun protection from my suitcase, before heading to the beach chairs after breakfast 😀😀😀.