Purpose :
To achieve a more realistic economic effect for nations engaged in hostilities with one another. It seems unlikely that enemy flagged trade vessels would be permitted to go about their business unhindered in ports controlled by their foes. Thus this tax could be viewed as the necessary bribe paid to port officials to 'look the other way' as their contraband goods are loaded and unloaded.
Proposal :
That a nation may impose a trading tax upon selected nations it may be in conflict with at all ports under its control. This may also be seen as some recompense for any insurance losses under-written by the nation due to actions of the other nation.
The level of taxation may be varied to reflect the intensity of animosity between the two nations, from a nominal level (1-2%?) to mark displeasure up to full on punitive level (5-10-15%?) to discourage any trade at all into their ports..
Effect :
This allows for an economic element to conflict : by varying the tax rate, nations can indicate their level of discontent with another nation ; absence of such a tax would indicate friendship and goodwill exists between them and encourage more trade between them to the benefit of both nations.