I don't remember there being a naval battle in the tutorial - at least, I didn't encounter one. I kinda got lucky - there was a full stack of units defending the target province, so I backed off to recruit more men, and in an attempt to gain some allies, struck a trade deal with the (Christian) clan to the north, in the (probably vain) hope that they'd help me double-team the target if I could befriend them. My enemy responded by attacking said 'future ally' to the north in what I can only assume was a freakish coincidence, leaving their capital practically unguarded. I laid siege to their fort, forgetting that their army was only one turn away and could easily march back to break the siege... except they didn't, instead heading further north. Next turn, I attack, tear them apart, tutorial ends.
Given that I was expecting to have to befriend a few allies, pile 20 units into a stack, and pray to the autoresolve gods, this was kind of an anticlimactic conclusion.
As to your problem... yuck. You could reload an earlier save, if you don't mind savescumming and actually
have an earlier save. Or you could rebuild your army, ally with nearby clans to provide additional firepower, and prepare for a massive, epic showdown to decide the fate of
this tiny portion of Japan!