Have wondered about water cooling my sofa but not sure how easily to get tubes you can lay on and pump water through and be comfortable to lay on. Cooling the water is fairly easy, ice cubes short term and heat pump aquarium chiller if you want it to run for a long time is probably the most reasonably sized/priced option.

Modifying an old fridge or freezer could also work if you drill a hole through it to have a bucket inside cooling down and pump water from that.