Can I block enemy's port if my navy is on the sea sector next to a factory zone?
I was able to find the answer after reading the rule book for 17th time after awhile.

Clearly the rules state on page 20 paragraph 2 in section restrictions on Placement

"Place sea units only in sea zones adjacent to territories containing eligible industrial complexes. New sea units can enter play even in a hostile sea zone. No combat occurs because the Conduct Combat phase is over."

So yes build your navy in the hostile navy sea sector adjacent to your factory but no battle is committed.