There are a couple of differences between pool and open-water goggles, depending on which you choose.
Pool goggles often don't feature polarisation for dealing with changing light conditions, and they're also more likely to fog up in murky water.
Open-water goggle lenses come in a range of tints, to cope with the different lighting conditions outdoors, and can be polarised or mirrored, while some are curved or faceted to increase visibility. They also tend to come with larger lenses, which therefore provide better peripheral vision.
Obviously a full open-water mask will elicit greater drag than a set of Swedish goggles. That said, this extra drag is fairly negligible compared to the other forces acting upon the body. And, if we’re honest, the Swedish-style goggles wouldn’t stand up well to a kick in the face!
The full wraparound goggle is a common choice for the open-water triathlete, and there are definite benefits associated with the greater peripheral vision and the larger distribution of pressure.
There are also potential benefits for iron-distance races – the generous volume of the wraparound captures more air, thus preventing the eyes from becoming oxygen-starved, as can be the case with smaller, independent lenses. By contrast, this larger goggle may interfere with your swim cap, can be more easily grabbed by your fellow competitors and doesn’t look particularly dynamic.
Your goggles might be one of your less expensive kit purchases, but it’s worth spending a bit of time thinking about your needs before you buy. Will you be using them to try and nail your ‘A’-race? If so, what will the conditions be like? If you’re heading to Lanza or somewhere similarly hot, then you’ll need a set than can cope with glare. If you’re targeting a UK race, however, or want your goggles to work for multiple different races, a set that can cope with mixed conditions from bright sun to murky water and cloudy skies will be better.
There isn’t a triathlon-specific goggle out there, but there’s certainly room for a goggle that can cope with the chaos of the start and the desire for speed during transition. Your final choice will probably be a compromise, based upon a combination of your chosen environment – pool or open water – and your personal preference in terms of vision, fit and size.