Jim,
I used to wear an old 2 piece beavertail 3mm, then went to a 6.5mm. It's still chilly below the themo. but you can always go back up to about 25ft. or so and you actually get rather warm after being below the thermo. A good bet would be a 5mm if you chill easily, then a core warmer and hood. The old layering thing. Just get down there once with whatever you have and check it out; you'll have fun and then you can decide what you need. I haven't gotten a hood yet but I think I will this year. Like Truckdriver said, those piranha perch can be vicious!