Although it is warm on Lake Baikal in summer, the winds here can be very harsh.
Be sure to take with you:
a sweater, a vest (optional), a windbreaker/raincoat;
shorts, bathing suit, a towel;
a hat, comfortable shoes to replace sandals, spare socks, sunglasses and cream.
If this is a hike, then take with you an additional medium-sized tourist backpack with a waist belt, comfortable trekking shoes, a KLMN (mug, spoon, bowl, knife), spray against ticks and mosquitoes, a personal first aid kit.
And a good mood!