Boats will collide with Marker armorstands, invulnerable, invisible, no matter what will always collide.

Boats also collide with paintings, itemframes, arrows, etc. Non-living (non-mob) entities.

What should happen:
Should only collide with "living" entities, boats, nonmarker armorstands, players, and such.
Not with marker armorstands, arrows, items, itemframes, paintings.

Code analysis by marcono1234 can be found in this comment.