Under the right conditions, you could do this without sacrificing anything. Usually the OpFor (opposing force) navy is in port until someone calls their admiral's cell phone to tell them that your ships have sailed. Yes, the whole premise is unfair, since ships move faster than any contemporary methods of communication.
If you know what port the OpFor is returning to, you can roughly know their travel time to get to your sea zone. This is where your tireless diplomacy (military access) can help you. You need to see the OpFor coming, so pick out a nation bordering a strategically crucial sea zone - one that the OpFor has to sail through to get to you. [If you're talking about England and France, that's an example where this doesn't work so well...] Then take advantage of military access to place a regiment, or a ship in that province. That unit is now your scout. If an ally controls such a province, that is even better. Since you're only trying to link up your ships into a larger navy, you need not do that all at once. So long as you have adequate military access, you should be able to "leapfrog" between ports. In each sea zone, there is going to be one port that is the quickest to dock at - it is usually closest to the center of the zone. If you have enough lead time before the OpFor comes to stomp on you, get to the next seazone and make port immediately. Once you are in port, the stupid AI will turn around and return to its port. ALWAYS make sure that you give the OpFor enough time to return and dock before you attempt to leave port again - or you will waste the very precious lead time that you have gained from moving a single sea zone. Continue as necessary until you rendezvous with the fleet, or get so cocky that the OpFor catches you.