Boat gets stuck underwater (submarine)

Hello, I was unsure where I should have posted this so I hope this helps.

I have been trying to make a log flume / boat trip for quite some time, to stop huge speed build ups ( they cause my boat to explode ) I used a trick where the boat will fall into more water, this was working fine in 1.7 but since 1.8 it appears the boat gets stuck underwater or just breaks. I have had no luck what so ever trying to get this to work, does any one have any ideas or videos they can share to build a reliable log flume?

This fails multiple times, but the reported bug is the 3rd attempt (around a minute in)

I have uploaded a video here:

http://youtu.be/IZeeNRTGAnA

If you need any more information, please let me know.