Obviously you would want a war immediately on Metal Casting. Just as Mongolia wants war at Chivalry.
But galleasses being scary? How unwilling to leverage extra movement are you? Beserkers don't need to end their turn anywhere near the coast. That's the whole advantage of disembarking with 3 movement.
The obvious use of Demark's UA is attacking coastal cities but an ambitious Denmark fan realizes they can attack near-coastal cities instead. Berserkers can swarm an unfortified but valuable near-coastal target, attack it from all sides next turn (with no river penalty), step backward, pillage, step forward, pillage, and attack it again on the third turn. City taken in 3 turns with no support. AI's city-connection income cut in half. AI reinforcements arrive completely out of formation and are easy pickings. Now attack the coastal city from friendly territory. This is how you use movement.
If you're being hit by ranged water units you're not trying.