To be honest, I don't think the campaign needs battle objectives at all. It seems simple enough to have tooltips, basically, in the campaign as to how to help your nation win a conflict, but the only goal that the campaign should have I think is to get favorable peace terms. How you do that should be up to you, but of course there are only so many ways a navy can influence a land war.
How you could influence the war overall, of course depends on the nations that are fighting. In the first campaign, for example, Germany could convoy raid out in the atlantic, and try to slowly starve the British war effort that way, but you should also be able to intercept/prevent reinforcements headed for the BEF, and that should be able to end the war much faster (but would require effectively total naval dominance).
There should be battle objectives you can select from in custom battles, but I don't think there should be battle objectives in the campaign.