IMHO, the port control and nation system is broken : currently there is little point in being a citizen of a country other than Barbados or Spain unless you are perverse or just like the idea of being a King of Nowhere : I don't say its their fault; they have, rightly, played the game as it is made : but as rational choice dictates new players should join their nations, it re-inforces their hold over 19 of the 21 ports in play, which is not what was planned when it was developed, I would suggest.
I think the situation demands an attritional rate of influence loss as the number of ports controlled rises rather than the current mode in which so much income is generated then seizing another port is made easier, not harder. Then ownership would become much more fluid and as the possibility of eventually owning a port becomes more achievable, more nations would make the effort. Currently, I consider this part of the game to be largely irrelevant to me and my nation, and that using an influence-related card outside of a specific mission demand is a total waste of turns and time.