If you are trolling for salmon, you'll have better luck at 3.5 to 4 knots. Just say in'.
I think I'd water the current batteries, charge 'em, and see what happens. They might surprise you with a bit more service while you think about it. If not, it will only cost you some distilled water and a little bit of time.
-Chris
Best advice in this thread, and the easiest advice, and the cheapest!