I'm prone to prickly heat. A few years ago whilst on holiday in
Greece I had a really bad bout of it. I had a cream from the pharmacy which was absolutely brilliant. I used this cream each time I had prickly heat or a similar rash until the tube finished. I then took it to my Doctor to see if he could prescribe this cream for me. He checked the ingredients and said that these creams are not meant to be used repeatedly and would make my skin more sensitive in the long run. I had noticed that it was happening more often. He also said that that cream would not have been prescribed in the UK and that medicines or creams from pharmacies on holiday should be used as a one-off until you get home, just to solve the immediate problem. At home I won't take any medication without reading the instruction leaflet, yet on holiday I just accept what the pharmacist tells me, which is a bit daft of me really.
The advice I was given is to go into the water as soon as I feel hot so that my body has a chance to cool down. You don't have to stay in, you can just go for a dip as apparently overheating causes the problem. This is preventative, you don't wait until the rash appears. He prescribed a hydrocortisone cream for me as well just in case.
His advice has worked for me as I haven't had it for the last five years when I used to have it every time and I go in July.