Killarny...yes a good 3 stage charger is necessary for your AGM's...you don't say the SIZE in amp hours of your banks but since you will be plugged in full time to the dock it is of little importance on a daily basis what size charger you use. Where charger size WILL be of importance is when you are on the hook and out cruising around and depleting those battery banks and need to re-charge at a dock or with a generator. I will guess that you have 400AH of capcity...which means you will have 200 AH's to recharge when off cruising. This will take 5 hours to replenish to about 90% with a 40 amp charger so an overnight stay at a marina will get you fully topped off. You can substitute your actual numbers for my assumption to see if it makes sense for you.
Living at the dock with the DC stuff in heavy use...you're still gonna be using less than 20amps DC so even a small charger will suffice to keep you topped up full time as long as you remain plugged in.