I hate this "War on X" rhetoric. They're not going to bomb the ocean or send troops to shoot it, so why make it sound like it? Just say what they're actually going to do about it.
Perhaps if they demonstrate a decisive victory over the forces of one Seaweed Lord, and they offer reasonable terms to the rest, they can win their vassalage without further bloodshed?
A strange case of nominative determinism.
As a tactic though it's a bit like tracers, they work in both directions.