Naval Battles

I
t's very similar to attacking with OLU. Each naval unit has one defense and one attack, except transport boats.
Use Light Ships in attacks.
Use Birremes in defenses.
Use Trirremes in conquest cities.
I recommend to focus in one type of naval units in each of your naval cities. Doing this, your attacks/defense will be more effective.
Basic tactics:

• Save your birremes to important defense like a city under a revolt(revolt world) or to support a siege(conquest world). This means you will dodge all the LS attacks.
• When you'll be offline send your LS ships along with your OLU to an inactive city far away from you, then if you're city suffer an attack the enemy won't kill your troops.

Put your doubts in the comments.