Not having goldfish, I have never explored this area, so my comments will be very general but I'm sure other members with knowledge in this specific area will be commenting.
Quite a number of plants would survive cooler temperatures, but goldfish do like to munch on plants, so the plants chosen would have to be tough and unpalatable. Java Fern comes to mind, I have never heard of anything that would eat this thought something out there might; and I have had good JF underwater in amphibian vivariums with no heat (room temp). Some of the larger swords (Echinodorus) are fairly tough plants, but again without experience in goldfish I have no idea how persistent they may be especially as they get larger. When I had goldfish in my outdoor pond, floating water lettuce and hyacinth seemed to do fine--until the raccoon, who love the latter and the fish, found the pond; all gone now.