Here is an idea.

If Britian was to surrender then is it not resonable to assume that teh Royal Navy would be incorporated into the Kriegsmarine along with possibly the French Navy. Know i may not be the best person when it comes to maths but it doesnt take a genious to work out that the two most powerful navys in Europe put under the control of the Kriegsmarine could do a hell of alot of damage to any attacking US fleet.

Then assuming the US would launch an attack around mid to late 1945 that gives the Germans alot of years to build a massive sub fleet, Several carriers and build up a strong defensive force.

They could end up with upto 24+ Battle ships alone by 1945.

Just a fought, Oh and also it means that the Russians would have to face the full industrial power of Germany instead of some being diverted to France and North Africa etc.