Which is the best land/sea-means to travel from Shantou to Fuzhou?

Well, there is no direct train or boat that can take you to Fuzhou.
But you can take direct long-distance bus at City Bus Station. The schedules are 09:40, 11:10, 12:00, 16:10, 17:10 and 22:10. The ticket fare is CNY 170 per person. It takes 6 to 8 hours on the way.

Thanks Daniel. Much appreciated. Are there any attractions en route?

Oh, it's a direct long-distance bus. There are just a few short stopover during the whole way.
So you don't have time to tour around some attractions.
However, seeing the natural scenery along the road is also a good choice.
Have a good journey!

Thank you again Daniel.