Koh Samui has plenty of beautiful beaches. Some like Chaweng https://www.kohsamuievents.com/locations/chaweng/
have a wide sandy beach with access deep water on most of it. Other beaches such as Bang Po is thiner and shallower. The high and lows tides will affect these beaches as you might find it harder to swim or no room for your towel to lay on the beach!
https://www.kohsamuievents.com/locations/bang-po/
Koh Samui is only 25km long and 21km wise and has one road that road that goes nearly all the way around the island. You have to cut inland in the North east where you find Plai Leam and Choegmon Beache and in the South East where you find Ban Talingnam. The North and East typically have the deepest beaches and the South the most shallow. Choegmon, Chaweng , Silver beach and Lamai have the finest white sands. It’s an absolute must renting a car or a bike and taking in as many of the beaches as possible. I’m still finding new ones and I’ve been here 20 years!
Here is an excellent guides to the tides on Koh Samui and some information about beaches and the waters http://true-beachfront.com/guide/gulf-of-thailand-strange-tides-high-low-tides-all-day-night