Unless your tri is indoors, most people will wear wetsuits for the swim. Unless you're used to pretty cold water (most likely under 70 degrees F depending on your location), I'd recommend getting one.
I wouldn't wear long sleeves for the swim under a wetsuit. Depending on the actual temperature, I'd probably wear my tri suit + a cycling jersey for the bike, and probably have arm warmers to put on just in case. I'd also take a light zip jacket just in case that would be the easier solution.
“Hope is the thing with feathers, that perches in the soul, and sings the tune without words, and never stops at all.”
- Emily Dickinson