My first sprint tri was in water at about 62 degrees. I didn't have a wetsuit and I was fine. I doubled up the swim caps but, other than tri shorts and a tri top, that was it. Do something active before you first get in to build a little internal warmth. Definitely get in for a "warm up" swim to get over the initial shock. My tri swim had a run start from the beach. After the "warm up" swim, I did a few short sprints and jumping jacks on the beach to stay warm while waiting for the start. Once the race started, I didn't even notice the water temperature.
I grew up swimming in Canadian lakes and rivers though so YMMV.
The silicon caps can be really tight when doubled up let alone tripled. My fav is a single silicon cap pulled low over my ears with a strap bubble cap over it. Warm and watertight and comfortable and the bubble caps are cheap and come in a zillion colors.
For some other cap ideas for cold water open water swims see