I find naval combat a bit too random to worry about whether or not to stack ships or whatever. Sometimes I clean up and end up killing 8 ships in one turn for a loss of one of my own, and other times I'm on the end of a beating (worse was when my Scilian fleet lost anywhere between 10 and 15 ships to the Italians and Byzantines in a single year). By the time I'm losing loads of ships, I'm usually filthy rich because I was master of sea trade beforehand anyway, and I start churning out more ships to replace those I've lost and before long it's 'I think you'll find I rule the waves Mr. AI and I don't care to share the seas with your little toy boats' type thing. The only thing I make sure is before attacking I have at least the same or more ships than the AI. Then I let the random maths do its thing.