for a province to have a port to be invaded
A province doesn't have to have a port to be invaded from the sea. However, it does need a port to let agents visit by sea though. I know it's weird.

As for the Jihad/Crusade, they can use other factions' fleets.

Sicilians launched a crusade to Algeria (they went around lol) and I wanted to destroy it as I was playing Byzantine. I sent many troops to Constantinople to repel the crusade but silly me didn't read the message that the Crusade is arriving in Nicaea instead of Constantinople by sea. They used Bulgarian's fleet (only fleet present there) to get across Black Sea.