Can someone clarify the blockade rules?

The rulebook says:

Place all of your new sea units in any sea zone that shares a naval base symbol with a territory that you originally controlled
(has your power’s emblem) and is not currently controlled by an enemy power.

Does this mean if enemy have battleships in your ports you cant mobilize new sea units there?
If so does transports and submarines count?

Or can i mobilize new sea units even if there are ships on the sea zone but I control the land zone where the harbor is?
If so when does the sea battle occur when I mobilize in a port with enemy ships?

Thanks