I’ve been asked about this by other optically challenged swimmers a few times, so I thought I’d post what I’ve emailed them. For prescription goggles, you need to get your glasses prescription from whoever is your optician. I just asked in Specsavers and got it no problem.
I took 27 minutes to go around a 750m course on my first triathlon. My wife videoed me going in the opposite direction to the course several times. I hadn’t learned to swim, but not having prescription goggles made it much worse. I still have problems with sighting, but that’s technique now rather than my eyesight. I’m close to blind without glasses so I’ve spent a bit of time on getting the right prescription goggles!
I paid a horrendous price to Specsavers for my first pair. They leaked. I got a second cheaper pair from them (still well over €100). They snapped. I took to the web. The first pair I bought via Google were from www.aquagoggles.com and these are like pool ones. Also available tinted, which the other ones I now use aren’t. I still have the aquagoggles and I’ve used them in the sea but more commonly in the pool. The strap is a bit finicky and the left one very occasionally leaks a little sometimes, but only if I’m not careful with fitting it. These ones cost about €30. They do a pretty good job.
Also, on the Aquagoggles site was the best guide to how to work out what your correct choice of prescription lens should be that I found:
The ones I now use are Aquasphere Eagles – not available tinted, which is a pity, but they have some huge advantages. They’re bigger, which is good for open water sighting I find, and they have interchangeable lenses, so that when they get scratched, you just buy new lenses and slot them in to the same frame. About €11 per lens. I’ve done up to seven hours with them and they don’t leak or hurt.
You buy the ordinary ones with clear lens, then buy the lens to match your prescription – very different ones for the left and right eyes in my case. The lens are available in half sizes for your prescription.
I originally bought the googles from a UK website, Wiggle, because I couldn’t find them in Cork, but they’re now available in Edge Sports and on their website and at prices to match the UK suppliers.
Hope this is helpful. You need to see where you’re going when you leave the pool and head for the open water!