River Rash

after being in the sugar creek river my friend has red blisters on his body and legs and arms they itch sometimes and sometimes not can you please tell us what they are as his sister got them too, I am the only one who didn't get them.
He has been home for two weeks now and they are getting worse,  

[Dr Santos]

Hi,

The red blisters may be due to contact or irritant dermatitis from plants, insect bites, or allergic reaction. What are other signs or symptoms present? In particular, exposure to poison ivy, oak, or sumac causes an itching rash that usually appears within 24-72 hours. The rash usually starts as small red bumps, and later develops blisters of variable size and may crust or ooze. The rash may be found anywhere on the body that has contacted the oil from the plant.  

Cool compresses, calamine and antihistamines may help with the itching. However, it would best for your friend to see a doctor especially with the length of time they've been experiencing this and with the worsening symptoms. It is also good to see a doctor if  any of the following is observed: large areas of rash causing significant discomfort; rash on the mouth, genitals, or around the eyes; area of the rash that becomes infected or drains pus; if there is swelling present or difficulty of breathing.

Take care and regards.