I am usually a big baby when it comes to getting in the water- in the pool or the ocean. I can tell you that a wetsuit makes a big difference. Of course you still have that initial shock when your wetsuit soaks up cold water. You can also use a neoprene cap if the water is ice-cream-headache cold, (mid to high 50s?), or wear two (or more I suppose) swim caps if it's too warm for a neoprene cap but still cold.

I said I'm usually a big baby, but I did manage to do a 3 mile ocean swim in 58 degree water with no wetsuit after my lungs practically seized up in shock. Brrrrrr! No wetsuits allowed, otherwise, I would have used mine for sure! Don't worry- if I can do that, you can do the tri in your wetsuit.