The national park authorities stick to middle of October for the 2015 season opening date of the Similan Islands.
With the additional two weeks of opening time, the park rangers are aiming for an increase in visitor fees. The middle of October marks the end of the rainy season with usually very good and calm weather conditions. The dry northeasterly monsoon usually kicks in around middle of November, bringing cool and dry air from the chinese mainland down into the Indian Ocean. At the beginning, the dry weather winds can be quite strong and build up some waves between November and January whereas middle of October the weather is mostly very calm. For this reason it is not in any way more dangerous to visit the Similans ealry in the season as the time with the most waves is around Christmas and New Years.