first thing - check if the organisers are supplying swim caps and whether they will allow swim socks (it's banned in triathlon and many OW swim events follow the same rules)
if they don't supply swim caps, neoprene caps are usually fine but are quite expensive for a one-off swim - 2 latex swim caps work out cheaper and will keep the nut warm
if you are allowed socks, then a few companies sell them - my missus has a pair of Blue Seventy ones designed for swimming. I use a pair of thin neoprene surf socks picked up at a local beach store - the kind that sell rubber rings, fishing nets etc.