Do you ever see the destruction that ships can bring? I will try explain it to you. It may be to my own demise, but if you don't know, you are only hurting yourself. Every unit no matter the kind, has a tiny red dot which shows position. If you place your ship red dot near shoreline, you can kill units during embark/disembarking. At sea you can go against other ships or a big favorite is hitting a vulnerable stack of units not held or protected in ships. Sometimes during war you can slither by a ship. which is why you need to be around cause you can't depend on a blockade. Normally you need to be around to order the strike.
Like being around for a strike you also need to be around if you wish to survive a ship attack. Ships work like land units, 1 vs.1. If in a stack, you can sacrifice a unit to satisfy the attack and split your units to push on towards your intended target. Time is your biggest factor especially if you are a troop. Harbors are your remedy for access , the higher your harbor the quicker than nearly 8 hrs to invade. I think at harbor 3 under 4 hrs. if target close. Best thing for stacks to do is individually send and bring stack later on land of intended target. There I shared. The rest is up to you.

Yes. You have to be there just in case they do slip by which is 90% chance they will. Just as miraculously yours may slip by as well. You have to order the attack. Sometimes you have to wait for red dot connect as well. I have been apart of many blockades in alliance tournament games and we learned the hard way to wake to alarms so we could be there in the nick of time to make sure we sank those ships:)