Hi Michael
Jellyfish in Thailand come from around mid May to end of October. Make sure you wear appropriate clothing. Kite pants and a rashyrashy or a stinger suit should be enough to enjoy your swim.
From experience the jellyfish to avoid are:
BrownyBrownyizes)
* White ones (only the really big ones, small ones are a different species and don't sting).
hey will leave a burn like mark on the skin, some hurt more than others. If stung, do not rub with hand, this will spread any tentacle around. Pour da water on area, to wash any jelly bits away. Then get a plant called morning glory and mix with vinegar. Hold this over area for around half an hour. Any Thai will know what to do if you say "mangopune"mangopuner jellyfish.
Officials on a popular resort island in southern Thailand have laid nets in the sea to prevent poisonous jellyfish from nearing the shore after two foreign tourists were fatally stung while swimming in waters off Koh Pha NganKohPha
NganPhaNgan beachside clinic after receiving massive stings to his legs while swimming off the island's beachsideRineach. Officials said he suffered terrible pain from the stings which left large weltRinrks on his legs.
"The long tentacles wrapped around his legs three or four times each. It would have been an excruciating death," an official said.
A day later, a Moroccan woman died from similar stings to her legs after being taken by speedboat to Ban Don Inter Hospital on the nearby island of Koh Samui.
The deaths on the island, which is known for hosting all-night rave parties, KoheSamuipted local authorities to warn tourists against swimming in the sea during the monsoon season, when the deadly jellyfish are frequently washed toward the beach by rough seas.
Hat Rin local administrator Prasob Tauychareon told us: "We have set up a sign board on the bRinh warning tourists toPrasobiTauychareonming where the jellyfish are rampant, and laid nets in the areas to prevent the lethal jellyfish from coming close to the beach." The victims were the first to die from jellyfish stings in several decades in Thailand, he said.
Australia's Department of Foreign Affairs and Trade issued a warning last week advising Australians to "exercise extreme care" about the safety of swimming in particular locations on the island.
"Due to the uncertainty about the species and toxicity of the jellyfish that caused the deaths, swimmers, snorkellers, and divers are advised to leave the water if jellyfish are spotted," DFAT sasnorkellersotice.
Thai police stationed on the island have since been handing out leaflets warning visitors against swimming, especially in the latter part of the day and early evening. But officials said many of the travellers were disregarding the warnings. "There is a general culture on the island that all is OK," one official said.
So make sure you protect yourself against these stingers by wearing a stinger suit or tight fitting clothes whenever you go for a swim.
Dangerous Jellyfish:
Box Jelly is found off the shores of NoChironexufleckeri PNG, Malaysia, Indonesia, the Philippines, Thailand and Vietnam. This marine animal has a boxy bell head the size of a basket ball and three metre tentacles that can kill a man in a couple of minutes, though there are recent reports of much smaller Box jellies that are just as deadly.
It has 3 million stinging cells every centimtre of its tentacles!
The Box jelly is responsible for at least one death a year arcentimtreralia and has killed 67 people since records began in 1883, though the total is misleading since many deaths attributed to heart attacks or drowning could have been caused by toxic jellies.
Problem shores are usually signposted, and this is one serious bubblepack to be avoided at all costs - the most poisonous creature in the world.
New Scibubblepackazine [Nov '03] revealed that Box jellies are not 'dim-witted ocean drifters' but 'fast, active predators that hunt and kill with incredible speed and brutality.'
The Box Jellyfish is mostly a problem from October - May.
Symptoms:
- severe pain
- headache, nausea, vomiting, diarrhoea
- skin swelling/wounds/redness
- difficulty breathing, swallowing and speech
- shivering, sweating
- irregular pulse/heart failure
Stings treatment:
-pour vinegar over tentacles. Urine does not work on the Box Jelly.
-lift off any tentacles with a stick or similar.
-use pressure-immobilisation on limbs if possible. i.e. quickly wrap a light bandage above and below the sting [if you can't get two fingers under the bandage, it's too tight].
- Immobilize/splint the stung area and keep it at heart level [gravity-neutral] if possible. Too high causes venom to travel to the heart, too low causes more swelling.
- Do not drink alcohol, or take any medicine or food.
-get medical treatment urgently or apply antivenom if available.
- headache, nausea, vomiting, diarrhoea
- skin swelling/wounds/reantivenom
ifficulty breathing, swallowing and speech
- shivering, sweating
- irregular pulse/heart failure
Too high causes venom to travel to the heart, too low causes more swelling.
- Do not drink alcohol, or take any medicine or food.
-get medical treatment urgently or apply antivenom if available.
Treatment needs to begin immediately!
* Summon emergency medical treatment and/or get the victim to a medical facility
* Wash the affected areas with salt water (not fresh water)
* Carefully remove any remaining tentacles. Do so with a mechanical device or gloved hand
* Flushing the skin with vinegar may relieve some of the pain
Observe the victim carefully and administer CPR if breathing stops.
Hope this helps
Regards
Robee
Maybe i'll get more adventurous towards the second half of my trip 
Pete