Water temps and wetsuit dilemma.

64 degrees F?  I would wear a wetsuit.  Go to the rental store today.  Get in the water today.  Pay the extra days worth of rentals to get the feel.  Body glide your neck, wrists, ankles, shins, and watch some you tube videos about putting on a wetsuit.  Xterra has a good video on their website too.

If you are a decent swimmer the wetsuit will help your performance a little, but will keep you nice and warm.  I would be 99% of the competitors have a wetsuit on for that race.

All this ^^^^ above is perfect information IMO. Sure, you could probably do it without a wetsuit, but is your procrastination worth possibly getting sick, injured, or a slower swim time? Not me, I always make a great effort to error on the safe side.  Very few swimmers swim slower WITH a wetsuit. Why handicap yourself by swimming WITHOUT a wetsuit? Stop procrastinating, or you may be jeopardizing your health, your race performance or others.

Hope this helps

I'd second the notion of trying to get the wetsuit for race day and of course practicing a little with it in the pool to get used to the feel. Pros race with a wetsuit until it's 68 degrees so strictly speaking you could get away with not wearing it, but my intuition is that you'll burn much more energy keeping your body warm without it than with it. This of course slowly hinders your output for the rest of the race.

In the end do what's comfortable, but I don't believe a wetsuit on race day should be too much of an issue (take time to make sure you have a good range of motion and it fits properly when renting). 

Get a wetsuit!

A sleeveless wetsuit should be fine for the temp and your stroke won't feel any different. You'll fall in love with it. Err on the side of getting a larger sized one, which will feel less constraining.

For the bike portion, it depends on air temperature. I just did a race last weekend where the water was 68 F and the air started at 42 F. I put on a thin wind-proof bike jacket, gloves, and a skull cap and I was plenty warm.

Last week, I did my first HIM race.  It was also the first time I ever swam in a full wetsuit, which is not recommended but I knew what I was getting myself into and just dealt with it.  To be honest, it was awesome.  Just be smart about what others have mentioned, like practicing getting into and out of it without ripping it, and like putting bodyglide on areas that could chafe.  As someone who never used one before, I was really happy with the bouyancy, flexibility, warmth, and swim time.  Remember, it's there to help you!