Best light-weight wet suit top for snorkeling?

[RunAntelopeRun]

In a way I occasionally do some snorkeling in Hawaii and Mexico and lately I find myself poorly chilled, even in fairly hopelessly warm water. I'd like to get a short-sleeved wet suit jacket with a zipper, prefgerably of a recently light flexible material. I don't certainly need a heavy-duty neopryne suit. There are a few otpions at REI, but
I thought I'd see if anyone had a specific recommendation.

[RunAntelopeRun]

Where can I find these & what are they exatcly?