You should really take the jellyfish in HuaHin serious - the brown ones. The big white ones are not so bad - the smaller brown ones are really dangerous and those really do leave scars - permanent scars . People wear Lycra to protect themselves from these jellyfish.
Here is an excerpt from somebody who experienced a sting from one of them.
https://forum.thaivisa.com/topic/71217- ... the-facts/
"Posted June 3, 2006
I was stung by a jelly fish in Hua Hin during the rainy season in June three years. I have encountered jelly fish throughout my life in various parts of the world and had never suffered more than a rash and moderate paid that was gone by the next morning without treatment. This time it was different. I never saw the culprit as the water was murky but I felt something like a severe electric shock on my thigh. I don't know if water morning glory, meat tenderiser, pee or vinegar, the popular folk remedies would have worked as I had not read anything about jelly fish stings at that time, so didn't try any of them. The pain remained severe for several hours and I went to a clinic in Hua Hin where the doctor gave me some cream to rub in and told me it would go away in three or four days. After a week it was getting worse and started oozing pus and forming scabs. I went to BNH Hospital and was told I had shingles and prescribed some very expensive cream and tablets which I used for a few weeks but they didn't seem to help at all. The jelly fish wound did not heal properly for three months and frequently caused bouts of pain while walking when pus oozed through my trouser leg causing a revolting wet patch. Three years later I have a scarred patch four inches across which is fortunately not too visible. I have never swum in the sea in Hua Hin again since that day and properly never will. The place I stay there has a nice pool so there seems no point in taking the risk of three months of pain and mess.
There are thousands of types of jelly fish and I have no idea which species attacked me. However, I am sure it is around Hua Hin in quantity particularly in the rainy season. Since being stung I have heard many other similar stories. A friend's young daughter has permanent scarring on her stomach as a result of a Hua Hin jelly fish sting and I heard of a young Thai woman who suffered a bad sting on her face.
To the poster who asked whether jelly fish could be a danger to his two year old child. I think the answer should now be obvious if you have read this. I urge you take good care of your child and recommend the use of a swimming pool while in Hua Hin."