I wouldn't be too worried. Look up Argentine's defaults, it was hard for locals with bank accounts frozen but not so difficult otherwise. People will not go Mad Max on the streets although the chance of muggings will definitely increase. As for infrastructure, if you plan a typical beach holiday and drink bottled water you probably don't really need a lot of it. You need a way to get around and I bet a lot of unemployed guys with cars will operate unlicenced taxis for that, just don't get mugged by them. Ideally, it would be best to travel as a group of 2-3 bigger male friends, you should be safe that way.

I was thinking of visiting the Greek islands this September. Is this looking unwise?

Public order will probably be fine, but the infrastructure might get problematic. Fuel could get scarce, hotels / shops / boat lines might go out of business, etc. If you spend your time in one place, it will be easier, but if you intend to move around a lot, there might be issues.