There are plenty of options going by sea and land from Helsinki to Munich. You need a lot of time, but they are perfectly doable and probably a bit cheaper than booking a new flight on short notice.
You can take the Finnlines ferry from Helsinki to Travemünde and then continue by bus or train. The ferry departs daily at 5:00pm and arrives in Travemünde 9:30pm the next day. If you continue by train, you will have to change trains and also spend the night in Hamburg. If you continue by bus, you will have to spend the night in Travemünde or Lübeck and take the bus with transfer in Berlin the next morning. Even short before departure, ticket prices are changing, but expect to pay about 120€ for the ferry and about 40€ for the bus.
You can also take a train to Turku and the ferry from Turku to Stockholm, which departs twice daily at 8:15am and 8:15pm. You write that there is an hour and a half drive from your cruise ship port to Helsinki airport. Perhaps your cruise ship is disembarking in or near Turku anyway. From Stockholm, you can then either continue by train with transfer in Copenhagen and Hamburg or by bus with transfer in Hamburg or Berlin or even a combination of train and bus. The cheapest option is likely to take the train from Helsinki to Turku (20€), the ferry the next morning from Turku to Stockholm (15€) and then continue by bus (Flixbus) to Munich (about 75€) for a total of 110€. The night ferry is more expensive, since you need to pay at least 70-80€ extra for a compulsory bed or cabin. Going by train from Stockholm to Munich is quite a bit faster than by bus, but tickets can be expensive on short notice, up to 340€ depending on exactly which connection you book.