Card: Scout
Cost: 16
Non-stackable, one cast per 24 hours per target.
Description: Select a player. Plunder attacks against that player's fleets have a doubled maximum ransom.
Maybe a percent chance to fail in case the target was to find your scout, but the card would still be used.
Alternatively,
Cost: 8+X
Description: Select a player. Plunder attacks against that player's fleets have maximum ransom increased by 2000(X).
Numbers in this case would obviously need tweaking as you'd have to invest over 100 turns/25 attacks to see any significant increase with the above, it's just a handy example. 50,000*sqrt(X) seems like it would be better with a convenient effectiveness cap at some point, but is a bit more complicated than most cards.