Mustering in port territories
Yes, that is correct. However remember that as soon as you have two or more ships in the same area (port or sea), they are considered an army and must adhere to supply limits.

Obviously if you muster into both the port and sea you're potentially blocking your port ship for being able to move out into the sea. That makes your castle/stronghold a ripe target for attack as if an enemy were to defeat that one footman, any boats in port would be replaced by an equal number of your enemy's ships (pg.25).

Something else to note: The special Consolidate Power token must be placed on the castle/stronghold in order to muster. You are not allowed to muster by placing the special Consolidate Power token on the port space.