there is a knack to using them, we have had them for a couple of years now and they are great, provided you do this fill them up open the shower bit and hold that in the air squeeze it till air comes out then close again, the water wont heat up if the sun is shining on black plastic with a big air bubble! and the best place to but it is on a dark coloured car this warms the underneath aswell, we have two bags and only fill them half way as they heat up quicker.
Took mine on holiday and came in really useful for using when the kids had sandy feet from the beach. Does heat up really well on the roof of the car too.
We used one of these the first time just a few weeks ago in the very hot weather. Attached it to centre awnig bar for my 8 yr old in his trunks he had a great time. Not much pressure of course more a gentle sprinkle!