There are a handful of good performances involved -- e.g., Stephen's Flint -- but by in large, the acting is terrible. Which is such a shame because the actual story is pretty decent. If you can make it through the first season, you'll be rewarded.
Besides all the Pirating action, I'm really surprised how credible the economics and politics of the show is. I'm often embarrassed by how naively those things are treated in TV, but here it mostly makes sense on at least some level.
Aside for 1 or 2 miscasts I find the acting pretty damn great. All the 4 captains are a joy to watch.
I did stop watching for a time after a really dumb plot development at the end of Season 2, but I'm glad I picked it up again.