Hat Sai Kaew is the widest, longest, and most developed beach on Ko Samet, on the north-east coast a short walk from Na Dan pier. The sand is genuinely white and fine — the best on the island — and the water is shallow enough for paddling a long way out before it gets deep. Beach bars, sun loungers, and food stalls line the shore in sufficient numbers that you don't have to bring anything but yourself.
It is also the most crowded beach on the island, especially on weekends when Bangkok empties out. If you want space, come on a weekday or early in the morning before the day-trippers arrive. The beach faces east, so the best swimming light is in the morning and there's shade behind the tree line by mid-afternoon.