I have done a few tris when it has been in the 50s. I will put on a bike jacket in T1 and typically take it off in T2. Full zipper makes putting it on wet much easier.

Putting on something before you swim it will be wet and you'll be cold riding.

Practice putting on your jacket or whatever while wet. I figured out how to have it over my bike seat so it was the easiest to put on. Seriously riding when it is 50 in shorts and no sleeves dripping wet is insane. With my wind jacket I was comfortable.