Someone just asked me about Krabi coastline and they'd "heard" swimming isn't possible because of sea lice, jelly fish etc. I've never been there but someone on here may know what this is about. Any advice?

I’m hoping by this comes under the ‘everything else’ category! Myself and a group of friends have just returned from a fantastic three week trip to Phuket and the surrounding islands. During the last week there about half the group were bitten/stung. We are trying to identify what it could be. None of us saw or felt what bit us. The first time you are aware you have been bitten is when you notice a small blister. The blister grows in size over a few days and is incredibly painful until it pops. Some of us have been put on antibiotics as the bites got infected. Also seems to cause an upset stomach but this might just be a coincidence. The bite is not itchy - so not likely to be a mosquito or sand fly I think. Does this sound familiar to anyone??