Sailors have a thing about winds, and to them their favourite winds are the ones that blow your ship to where you want to go, normally your final destination..the port.
However, if you happen to be a sailor who speaks Latin, then you will describe these winds as ob portunus, or towards the port, because Portunus was the god of harbours. These ob portunus winds are good and favourable and represent, for the homesick seasick sailor, an opportunity.
By a reverse of this process, you can really pick the wrong moment to ask somebody for something and thus you are im-portuning them.
Etymologically speaking, this has a rather pleasant side-effect. Port wine is named after Portugal which is named after the Portus Cale. So if you hurl yourself towards an unguarded bottle of port then you have become an opportunist.
When leaving port, you should rely more on the tides than the winds. Wait till high tide, or flood, and then let the retreating waters pull your boat out. To see exactly how the metaphor survives have a look at this passage from Shakespeare.
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
On such a full sea are we now afloat;
And we must take the current when it serves,
Or lose our ventures.