It's hard to say what they mean about the "Sea Dagger", it seems to be a melee ship, but that's not unique. Does it gain extra gold from taking coastal cities?
I'm with you here. Might be that whoever wrote the article doesn't know it better .
The 'Sea Dagger' may be able to build Polders on Coastal Tiles?
I think that's two clauses in a not greatly written sentence.
They have Sea Daggers
They can build Polders which...
ETA: Any Dutch person have a clue what a Sea Dagger is?
I understood it in the same way like Louis, I changed my translation a bit .