You can tell a good waterproof cushion from a bad one before you even sit down. The good one holds its shape, the fabric has a slight stiffness or a coating you can feel under your thumb, and water sits on top in beads instead of soaking straight through. The poor one looks the part from across the patio, then you press on it after a light rain and your hand comes away damp, because the cover is only water resistant and the foam inside has already started drinking.
Where these go wrong is less about the cushion and more about what is near it. Keep them off raw wood furniture that is not sealed, since the moisture trapped underneath can mark or warp the surface over a season. Skip anything busy with metallic thread or delicate trims, because those details are usually the first thing to fail when the fabric gets wet and dries out again and again. And if you are drawn to a deep color for a shaded porch, go in with your eyes open, since some darker dyes fade patchier than lighter ones once the sun gets at them regularly.
Honestly, these are not the cushion for a covered, bone dry sunroom or a formal indoor adjacent space where you want a soft, natural feel under your hand. The foam core, even the good quick dry kind, has a slightly different give than a regular cushion, a bit more bounce back, less sink. If you want that plush, sit down and disappear feeling, this is not where you will find it. Over time, the honest ones age well if you treat them simply: hose them off, let them air dry standing on an edge rather than flat, and bring them in or cover them for a hard freeze or a long stretch of rain, since constant soaking is what eventually breaks down even a good foam. A quick wipe after pollen season keeps the color from staining into the seams.
For sizing, measure your actual chair or bench seat, not the cushion box, and account for an inch or so of overhang so it does not look perched. Standard patio chairs usually want something in the eighteen to twenty inch range, but built in bench seating runs bigger, and people guess low there more than anywhere else. Lay a tape measure across the seat at its widest point, check the depth front to back too, and if you are furnishing a whole set, buy one extra cushion so a single ruined one does not throw off the look of the rest.
