Swimwear, one or two sets in case you want to change , it´s not very pleasant to sit in your wet swimwear whilst having lunch.
A Hat or Cap is also quite handy if you want to protect your face from the sun.
Sunglasses are a must have as with the white beaches and crystal waters, its quite tiring for the eyes to be exposed to this most of the day.
A Sarong is quite useful to have. Even though most boats provide towels you cant really take them to the beach as you want to avoid losing them or bringing sand on board the boat, so take a sarong or extra towel if you are planning on staying on the beach as well.
Flip Flops or sandals is the most comfortable foot wear for the boats, and also useful to take with you when you get off the boat to go to a restaurant as you will be asked to take your footwear off when you are actually on the boat.
A beach bag is quite good to take on board as you will find it useful to take your wallet , phone or any extra things you want to take when going to the restaurant. The crew stays on board while you are having lunch so it is safe to leave things on board as well.
Clothing: Its good to take a long sleeve or sarong with you as when you are exposed all day to the sun and come into port late, (especially when its not peak season) it can get a bit chilly on the way back so its good to have something to cover up if you are feeling cold.