The longstanding joke about Pearl Lake is that the first five divers get 50 foot visibility and everybody else gets 5. It doesn't take much to stir things up there but it's a good training pit.
As far as temps go, look for something in the low-ish 50's. Temps will vary wildly this time of year - with the warm weather we've been having it could be a fair bit warmer than that.
A 6.5 mm suit is going to be cold but most folks find it tolerable. Literally hundreds, if not thousands, of divers dive the Pearl wet every year, most in 6.5 or 7 farmer johns. Bring something warm to drink and try to get out of your suit and dried off during the surface interval. Bring heavy sweaters and something to break the wind.
http://www.pearllake.com/frameset.htm
Think of it as a rite of passage - it's just something that needs doing and worrying about it won't change a thing. Prepare for the worst, hope for the best and go determined to have fun - you'll do just fine.