Has been a while since I made that trip, but IIRC the ferries between Finland and Sweden give discounts if you have an Interrail ticket. And they're not that expensivr to begin with, at least if you don't have a car with you.
I'm now living in Berlin instead of Helsinki, and I take trains to go pretty much everywhere in Europe. When you factor in the time of getting from/to airport, security checks etc. it is often nearly as fast and much more comfortable. As I write this, I'm having lunch near the railway station before leaving for a conference in France. Usually I'd eat in the train but now I happened to have the time to spare...