The extra weight is adding to the size of the wake and my local marina says that deep cycle batteries will wear out the stator on my outboard.
There's a lot more to it than your marina making this ^^^^ statement. Your engines regulator/rectifier prevents the deep cycle battery from gaining a charge >14+volts. Meaning, as you're running the engine the stator supplies charge back to the battery but never more to burn out stator. Your marina is referring to engines without the capability of regulating charge.
A deep cycle battery can be used for cranking the engine, so long as this battery is isolated from all other loads. You don't want to regularly discharge a deep cycle more than 20%-50% of total volt capacity, as it's cycle lifetime will decrease. Also, deep cycle batteries will have less cranking amps compared to typical starting battery of the same size. Deep cycle batteries are intended to be recharged over a extended period of time (6-12 hours), verses being topped off with charge by quick charging.
If you're running any other electronics on your boat, such as navigation lights, stereo, etc., these loads should be isolated to their own house battery. This application is best for deep cycle batteries. So you could do away with 1 of the 2 deep cycle batteries, buy a typical marine cranking battery, run engine load off cranking battery, and all other electronics off deep cycle.
I understand the premise of shifting weight forward to lessen the wake, but the weight of 1 battery will have very little effect on a 20ft boat, if any. Typical marine battery weighs 40-50lbs. Shifting both batteries forward would have the effect you seek. But I would not recommend going to only 1 battery IF you are running accessory electronics. Not a good idea, though many do it. Ironically, these are the same boaters who find themselves stranded because of a dead cranking battery.