Hives from heat - anyone else?

We go out to the arizona desert on the weekends to boat and have fun in the sun. DD has been going since she was 2 weeks old. Just this summer she started breaking out in hives typically on the last day of the weekend. It's usually 95*-105* out there but we stay cool by playing in the water.

 The 1st few trips this summer I thought it was a sunscreen allergy (which it very wel could also be) so we went thru every sensitive skin brand and had good luck this past weekend with California Baby but sure enough Monday (the hottest day of the weekend) she broke out in hives all over. Then it usually takes about 3 days after we get home to clear up.

 Has anyone else experienced something like this? I read a lot about heat rash and am fairly certain it's got to be what's causing the hives. I'm going to purchase some UV long sleeve swim shirts so she doesn't have to wear sunscreen and help keep her cool, anyone have experience with those? TIA