A very annoying thing is that boats can't go upstream, they can only go down.

This is a different issue than boats not floating when placed underwater. Even on a very gentle slope, boats cannot go upstream, which is a significant playability issue.

This issue completely prevents non-flat boat courses becuase boats can't go up even the slightest incline.  At all.  The issue is so bad that some players resort to building things like redstone piston stairs to get boats to go up, or a course where boats go into the nether to go up. Only player has to leave the boat for it to go into the nether.  

In 1.13, bubble columns (with soul sand) allow boats to go upstream.