I have no idea about the time period my function declined, but I only felt tired/low on energy the week I was admitted to hospital ready to start dialysis. My theory in my case is that the decline was over a number of years and my body adapted, with me never knowing it also wasn't in the back of my mind that I was no longer as fit as I was and put it down to hitting my 30's.
For swimming and a rare bath I was given activity pouches by the renal unit, these are just stoma bags (colostomy bags) so I didn't fancy messing about at the pool washing hands, finding a clean spot in the changing rooms etc to fit/remove one.