Short answer: the three coatings/features that matter most in a prescription swim goggle are a genuine anti-fog coating (not just a marketing sticker), UV400 protection for outdoor and open-water swimming, and a shatter-resistant polycarbonate lens — all three are standard on quality Rx swim goggles but frequently missing or degraded on budget ones.
Fogging happens when warm, humid air trapped inside the goggle condenses on the cooler lens surface — and for a swimmer who needs the goggle to correct their vision, a fogged lens defeats the entire purpose of wearing a prescription pair in the first place. Genuine anti-fog treatment is a hydrophilic coating applied to the inner lens surface during manufacturing, and it does wear down with repeated towel-drying and touching — which is why swimmers are advised to air-dry goggles rather than rub the inside of the lens, and why anti-fog performance is one of the first things to check when comparing two goggles that otherwise look identical.
Pool swimmers indoors don't need UV protection, but anyone swimming outdoors — open water, a rooftop pool, a beach session — is exposing their eyes to the same UV radiation a pair of sunglasses would need to block, with the added risk of reflected glare off the water surface. A UV400 rating means the lens blocks essentially all UVA and UVB radiation, and it should be a non-negotiable spec for any Rx swim goggle marketed for outdoor or open-water use.
Polycarbonate is the standard lens material for swim goggles because it's genuinely shatter-resistant, lightweight, and can be ground to a prescription power without the added weight glass would bring. It's the same reason polycarbonate is the default choice for sports and children's eyeglasses generally — impact resistance matters more in an active, wet, poolside environment than it does for someone wearing glasses at a desk.
Even a perfectly ground prescription lens only corrects vision properly if the goggle sits centered and sealed against the eye socket — a leaking or shifting goggle moves the optical center away from the pupil and effectively changes the correction being delivered. Silicone gaskets that seal without over-tightening, and an adjustable strap that holds position through a full swim session, matter as much to actual visual clarity as the diopter number itself.
Every pair in Salvador Eyewear's prescription swimming goggles range ships with anti-fog coating and UV400 protection as standard.