You can only hold out so long when in water that is cooler than your core body temperature. Hypothermia sets in...your mind gets impaired, you lose conciousness, you die long before thirst becomes a problem.
Ever go swimming for a couple of hours, and notice that your lips start turning bluish purple? Early signs that you better get out of the water and rewarm...
My mom learned Drownproofing from Fred Lanoue himself. It really does work - the "final exam" involves being tossed into the pool with arms and legs bound, as well as being tossed in fully clothed. You can use your pants or shirt (especially blue jeans) to rig an improvised PFD, all sorts of other clever ideas. You don't need extensive survival training, most of it is just common sense knowledge.
Most drowning deaths are due to panic not hypothermia.
I've been overboard in colorado river rafting. Its actually worse than you describe. For a while, you are actually paralyzed to some extent for a short time. I couldn't even get into the boat without help. But, I didn't panic, and that was the key. Of course, I was wearing a life jacket, but I found myself swimming anyway. I had to stay with the boat.
However, the water in this lake was nowhere near as cold as it was in the river I was in. It was probably in the high 60s in this lake and upper 40s in the river I went into. In addition, relatively, it wasn't that bad, as the temp wasn't in the 90s, but probably the low 70s. I've been in 65 degree water when its like 67 outside, and while the water feels cold initially, you get used to it pretty fast. On the other hand, water that's 75 degrees in 95 degree weather actually FEELS colder and takes a bit longer to get used to.