Does it have good transportation links with the rest of the land? (This is mandatory and cannot be waived.)
Does it have a natural port?
Is there an active economy behind it to be served by the port?
If it doesn't have a natural port, then is there a very active economy behind it which would justify building an artificial port?
That's all.
(If you do not need a "bustling metropolis" but are satisfied with a moderately successful city, then the requirement for an active economy behind the wannabe port can be replaced with having a strategic position, where ships must pass.)
(Also, note that a great port does not have to be on the coast; for example, neither Antwerp nor Hamburg are on the coast, and this does not prevent them for being among the largest and busiest ports in Europe. What's important is that sea-going ships are able to come to the port, for example on a river or on a canal.)