Since Sharks aren't a major problem in Thai waters, Jellyfish should be your main concern. Although they are fascinating to look at, their stings can be extremely painful, and one would require weeks to fully recover from them. In many cases, a jellyfish sting can leave behind brutal scars. Jellyfish stray away from the sun, so it is best to swim during sunny weather. Avoid hitting the water during overcast or rainy days. If you ever get bit by a jellyfish, a vinegar bath is the best and probably the only solution. If stung to a great degree, it can even lead to death if not treated well.