Usually the Italians and Sicilians dominate the seas early in early campaigns - they start with ships, ports and shipwrights IIRC.

Indeed the French are your primary enemy - the one you have to erradicate and supplant before doing anything else.

Remember that the cost of maintenance of ships increases linearly in direct proportion with their distance from a home port.

For example if your only port is say in Flanders, a ship in the English channel costs 20 flrns. A ship in Bay of Biscay 40 flrns. A ship in the Spanish cost 60 florins etc. This means that for really long chained networks of ships the cost can rise very quickly. The best way to reduce it is to conquer ports on the way to whatever provinces you are trading with.

With the above example in mind, if you have a port in say Leon in the Spanish cost (ie having conquered Leon and captured/built a port), the ship in the English Channel will cost again 20, the one in Bay of Biscay 40 but the one in the Spanish cost 20.

If you do the math for really long trading distances, say Flanders to Egypt you'll see that conquering say ports along the way for example in Malta or Crete or Sicily, radically reduces the meintenance costs (and hence increases the profits) of trade.

Also notice that all trading goods aren't of equal value - some are far more valuable than others, such as silk and spices as opposed to hides and wood.