Sounds like an interesting trip! When are you planning this?
Viking has daily departures on the route all year. Every morning, the ship Viking Glory leaves Turku heading for Stockholm and stops in Mariehamn just after lunch. And it's a pretty nice ship, here are some views of the interior: https://www.youtube.com/watch?v=2855Q13VuAw And in the summer, Silja also has ships on the route.
It takes a bit longer than flying but is worth it, and the ship is not a bad place to have lunch and watch the archipelago pass by. After a day on Åland you can board the ship again the next day and continue to Stockholm. Or take some other route to Sweden, there are few other ferry routes.