i think the issue here Danik, has to deal with the fact that there are ways to play plantations with no fleets, thus no sea battles.
So maybe the more important question needs to be how to make more fleets sail? Maybe no player market? Maybe more "petty" voodoo as you call it? If a person were so inclined, they could have plantations to pay for witch doctordom... with no way to "attack" that player other than turns.
Possible actions could be
- Player markets floor price notably higher than the cost of shipping good in yourself. Maybe the COGS = Revenue. Thus if you use only player market, you make 0 dollars from planation.
- Port Taxes
- Planation voodoo to attack production.
I personally like the COGS = Revenue. It would mean the more different types of goods you ship in the more profit you make

but if you just buy from player market, it will cost you 500k in goods to make 500k in revenue.
Maybe in this model, your guardhouse turns into a planation warehouse instead. You can only fill that warehouse from fleets. Your planation will always try to use that warehouse 1st for production and you get full profit, but any goods you have to take from normal warehouse get the % modifier to start decreasing your profits.
Remember...
You can sheer a sheep many times, but only skin it once!