I sleep on my 14' and 18' raft every chance I get. In the summer it's usually cooler "on" the river then up on the beach. My 18 foot raft has a 5 bay frame with two covered full bays in front of the captain's bay, room for two people.
If it rains, I set up a small two man tent, REI Camp Dome 2
, over the bays and clip it in to the perimeter line. I love a camp next to a rapid to drown out the snores coming from the beach. The rocking motion is usually soothing but at a couple of camps it can be excessive, I try to avoid those camps.
On the 14' I set a cot up over the front bay, if it rains, I'm headed to the beach.