Thanks for using this forum. As per the description given by you , it seems like from the exposure to prolonged water from swimming and hot bath after which the area of the crotch would still have remained moist there is simply a fungal infection
and prominently present in the hidden areas such as the groin and the thighs. You should avoid wearing the slips beneath and wear loose clothing. The main aim is to keep the area dry and in contact with air. If this persists then consult with your GP who can prescribe for you antifungal medication. Hope this helps. Regards.