Apparently it's not painful at all. The shock wave travels through the body doing it's internal damage faster than the nervous system message can get to the brain, so said one report on the Erebus accident. I'm not about to test the veracity of the statement though.
In my other life as a Reservist in the Canadian Armed Forces, one of my friends was a US Navy A6 Bombadier/Navigator. They use to fly up narrow valleys in cloud with only his Radar to keep them hitting the rocks.
I asked him what speed they were going and the answer was "usually over 380 kts". The significance of that was a Navy Medical study had concluded that if you hit at over 380 things happened so fast you never felt a thing as the nerves could not get the message to the brain before you were reduced to goo.
A friend of mine who owns a Lake says, "Sooner or later you will prang a seaplane." Even if you fly into the same lake every day there can be barely hidden rocks, or tree tops in the case of reservoirs (which most lakes round here are).
I do not agree with this quote. I know lots of very high time float/seaplane pilots who have never had an accident. The difference is that flying off the water demands more personal discipline,attaining/maintaining pilot skill and good proactive pilot judgement than going airport to airport.
If you are willing to make the effort there is no reason flying off the water should be more dangerous.