Yxklyx: Yes.
You mean: An enemy army is besieging one of your or your allies/vassals provinces, and you put a boatload of troops into the port? Then yes, they get a terrain bonus just as if you were attacker. It's possible, but I'm not positive, that you also have a penalty of sort of 'river crossing' from attacking from the sea, but I don't see anything on the wiki to support this, it might be something I've imagined.
You mean: An enemy army is besieging one of your or your allies/vassals provinces, and you put a boatload of troops into the port? Then yes, they get a terrain bonus just as if you were attacker. It's possible, but I'm not positive, that you also have a penalty of sort of 'river crossing' from attacking from the sea, but I don't see anything on the wiki to support this, it might be something I've imagined.