Water proof panties, in adult shopping, usually means a plastic, rubber, or vinyl pant worn over a pad. The cover stays on the body and keeps fluid off trousers, a skirt, or the seat of a car. A fashion panty with a thin cloth gusset is built for a small spot of moisture and lives on another shelf. A bed pad lies on a mattress, a chair, or a car seat and has no waist. The notes for this piece do not include a checked plastic, rubber, or vinyl pant, so you will not see a brand, a waist size, a count, or a price for that cover here.
People reach for the cover when a pad’s own backing has not kept a chair dry, or when they want a second layer for a long sit. The pad still does the absorbing. The pant keeps what gets past the pad off fabric you care about. Read the material line on whatever listing you open. Plastic, rubber, and vinyl do not breathe the way a cloth pull-on does. Change the wet pad. Sitting longer because a cover is holding the leak against your clothes is how skin gets angry.

The pad that goes under the cover
Shop the cover, when you find one, the way you shop any pant: waist and leg opening, measured where it will sit. Snaps at the sides let you put it on without stepping through, which helps if balance is shaky. A pull-on plastic pant means lifting each foot. Either cut can feel warm and can make a little noise when you walk. If the photo looks like ordinary cloth underwear, you are probably looking at a fashion brief, even when the title says waterproof.
Name the pad you will wear inside it. Poise Incontinence Pads, Maximum Absorbency, Long, are a 64-count. The listed length is 14.3 inches, and the page says each pad is individually wrapped. A long pad needs the underwear or the cover to be roomy enough for that length to lie flat. If it bunches, the extra inches do not help you.

The listing is Poise Maximum Long, 64 count.
Prevail Daily Pads for Bladder Leaks, Maximum Absorbency, are listed at 13 inches, 39 count. The page says they are individually wrapped and latex-free. Thirteen inches is a little shorter than 14.3. Lay a tape in the garment you will actually wear and see which length has room to stay straight. Both notes describe daytime bladder pads. Neither one records an overnight hour claim, so this page will not call them night pads.
The listing is Prevail daily pads, 13 inches, 39 count. Prices those listings showed when this article was written were $46.95 for the Poise 64-count and $19.36 for the Prevail 39-count. Check Amazon before you buy. Using only those figures, the Poise pad comes to about 73 cents and the Prevail pad to about 50 cents.
Bed pads, which stay off the body
Disposable underpads and washable bed pads protect a surface. Medline Ultrasorbs disposable bed pads in these notes are 30 by 36 inches, 70 count, with a backsheet the page describes as non-liquid-permeable, meant to keep a mattress or another surface dry. The listing showed $121.00 when this article was written. That price is for a case of large flat pads. A 30 by 36 inch pad has no leg holes. It will not stand in for a pant.
The listing is Medline Ultrasorbs, 30 by 36 inches, 70 count.
A washable pad in the same notes measures 36 by 76 inches, sized for king and California king beds, with tuck-in sides. The listing showed $39.99 when this article was written. A smaller washable pad, 17 by 22 inches, showed $9.65. The small size can sit on a chair or under a hip in bed. It is still a flat pad. Check Amazon before you buy either one.
The king-size listing is washable bed pad, 36 by 76 inches. The small listing is washable pad, 17 by 22 inches.
Disposable and washable bed sizes are compared here: Medline and washable bed pads. Use that page if the wet spot you are solving is on the sheet. If the wet spot is reaching your clothes, come back to a pad you can wear, then read the waist on a plastic, rubber, or vinyl pant before you order the cover. One cover in the right waist beats a stack of mattress pads that never touch the leak you had on your clothes.