Put a metal sign on a hinge that is at the too-tall height several hundred feet in front of the tunnel. When they hit it, they will definitely stop, and replacing the sign is a fraction of the cost of the water sign, which they can drive through anyway.
Other comments seem to suggest this doesn't work [Low bars etc..] I am not sure how large a problem this is [No audio on vid :(] Asking people to stop sharply on a suddenly wet road doesn't seem an ideal solution.
I think an addition would be to charge the drivers the cost of repair, this is not a tax on stupidity - it is a lesson in taking responsibility in driving an extra-ordinary sized vehicle on a public road.
Thankfully my driving instructor knew the importance of taking responsibility and understanding what that means before you get behind the wheel.