Well, you wouldn't want them completely flat and leaving you stranded in the middle of the lake.
But I was wondering how long it was before you noticed the batteries getting weak?
Some other conversions of the Akizuki/Hatsuzuki speak of the RC components being too heavy and/or the actual waterline being too high. Is that a concern with yours? I mean as a motorized model (motor/battery/gearbox), it was probably ideal as-is but one is adding considerably more weight with a servo, linkage, brass prop shafts and props and receiver.