With the new 1.9 boats, they no longer float to the surface when they run into a taller block of water. As such players can no longer create and use Water Elevators that use boats AND boats can no longer traverse up small river inclines used in man-made rivers.

I did some experimenting and found that these new 1.9 boats will only stay above water if its within a small layer at the very top of the water blocks, when pushed under this invisible "layer" that determines if it floats, the boat then sinks and promptly boots you out of the boat. While this does feel more realistic the problem is that Minecraft water is nowhere near the same as real water. Take the screenshots below as an example: Shown was a boating river race course I was designing that had the beginning stages forcing the player to boat upriver with non-steep water inclines of 1 block so they could go upriver to the top of this island then do a downriver segment later on.

In the screenshots I hopped into a boat to test, boated towards the first incline section of my course and instead of floating to the top of the river (a 1-block high "waterfall") it drove right through the rising river section then began to sink/boot me out.

I understand somewhat making realistic boats, but the inability to float up water blocks in these new boats kinda ruins most of their new upgrades' potential. Water/boat elevators and log flume rides aside the inability to rise up water blocks with this bug (if it is one) makes players very restricted on building things like boat racing tracks, which now cannot go upriver anymore.