The lower the number the slower the sink rate. A type II is relatively slow sinking line.
I don't have experience trolling streamers from a boat (done it in a float tube) but if I had to make an educated guess ...
A II should get you down 3 to 8 feet depending on fast you troll and how much line you let out. It's a full sinking line vs just a sink tip so the more line you let out the deeper it will sink ... up to a point. You could get down a little deeper by using a long leader (12') and weighted streamers
For shallow sub surface trolling I think it will work just fine.