Thailand is a hot country in general, although it is a good idea to pack some hoodie or light jacket protecting you from the wind (especially if you plan to enjoy many water activities like diving/snorkeling etc.).
Comfy flip flops or sandals should work well either for hanging out the beach or for sightseeing and short treks.
Hat or cap or bandana will protect you from the sunstroke, so do not forget to take one with you!
Necessary drugs/medicines (such as painkillers, electrolytes, diarrhea illness, disinfectants and patches for wounds) are available in every single store (7/11, Family Mart, and so on). Also, the mosquito repellents are easy to buy, so bringing them from Europe in our opinion is pointless although it is a good idea to bring the sunscreen (the one which is not affecting the coral reef though!), as the ones available in local drug stores are quite pricy and most of them contain whitening components.