HI KY caregiver, we have a water repelling mattress cover which has actually worked during a recent incontinent experience (apparently peed 3 times that night). I have the water repelling mattress cover, a washable quilted incontinence pad over that and under the sheet, a washable incontinence pad over the sheet (under dh) and dh is in pull-ups.
Many use a plastic mattress cover which zips around the mattress. I found that too hot because it doesn't breathe and it makes crinkling noises (at least ours did) in conjunction with some other form of pad either disposable or washable like the ones I have.
I got a plastic fitted mattress cover from Wal-Mart then put a quilted mattress cover over it. Then 2 30x36 disposable pads, then the sheet. On top of it all 30x36 washable pads. We seldom have to wash the quilted mattress cover.
I did much the same as LFL and Mary, but I used a shower curtain liner under the padded mattress cover. It was smoother and didn't make the crinkle noises. Best of luck finding what works for you.
I have a plastic fitted mattress cover then a mattress pad. On top of that is the sheet. I then have a 20x36 with wings washable pad on top of the sheet and a 35x80 waterproof pad on top of that. I found that if I only used the 20x36 pads, he would pull them out from underneath himself. Now I usually only have to change the big pad in the morning. He wears Abena M4 tabbed diapers at night and Tena for men pull-ups during the day.