One of the concepts of deep-frying a turkey (other than decreasing the total cooking time factor and flavor) is to seal in the natural juices ... so brining isn't necessary.
As for brining in general - look at the label on your turkey. If the ingredients say anything other than 100% natural turkey and nothing else (ie. if it includes any mention of sodium, water, broth, etc.) it should not be brined, and I would be a little leary of injecting them with anything containing salt - even if roasting in the oven. They have already been pumped-up, plumped-up and injected with all the salt and water they need. This also goes for "Kosher" turkeys.
Uncle Bob was right about measuring the oil you need before you start. And, when you lower your dried off turkey into the oil - do it slowly! You might also need to do it in stages ... lower it a couple of inches ... wait a few seconds (if ithe oil boils up too much pull it back up out of the oul and wait for the oil to settle down) ... lower a couple more inches ... it depends on the "boiling up" of the oil.
But, since you do fish frys - you've probably got the routine down.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain