The battle is for the treasure islands. You have to defend your own island and conquer your opponents island. On the boardgame you have tropical islands, reefs and treasure islands.
Filibuster is a loosely pirate-themed game where players take turns to move black and white plastic ships across a 13x13 square tile board. Apart from sea tiles there are skerry tiles (that cannot be entered), island tiles (which offer protection) and two treasure island tiles (which are the objectives of the game).
Each player has ten corvettes and three galleons at their disposal. Corvettes can land on islands but cannot sink other ships. Galleons, conversely, cannot land on islands but can sink an enemy ship by entering its tile at the end of a full 5-step move. The game ends when one player lands a corvette on the other player's treasure island.
Although there's no explicit art credit, the board bears the legend "Tegnestuen Den gyldne nøgle", Danish for "Studio The Golden Key", the firm name used by the Pagermans (professional artists both). The cover painting, however, showing generic pirate ships closing for battle, has a clipped, unreadable signature that doesn't seem to match either of their names.