This Happends Because When You Go On A Minecart/boat the block on top doesnt make you get out of the minecart/boat so it lets you see open spaces(Mines,mineshafts,ravines and dungeons) For boats Its Different It Only works when you shift off the boat because you get put on top the blocks let you see again open spaces(leaves dont know if others blocks work glass probably wont work as you can see trough it).

Hope This Helped Out.

-ButterMaster0315