I haven't towed a boat but I have towed a trailer, 5th wheel, and I did not trust the mfgrs. stated height. Nor should you. YOUR RIG may be slightly different from all others.
We travelled from Gibsons, B/C/ [Vancouver] to Indio initially and then other areas.
I measured it attached to the truck since that is the condition I needed to be concerned with. This was done on a flat and reasonably level parking area.
I simply got a long enough 2 x 4 and laid it on the highest point with a level on it and then dropped the tape over the side. Clamp the tape hook to the 2x4. I pulled out a bunch of the tape and locked it and let it over the side.. My wife pulled the tape to the ground and read off the measurement, BOTH SIDES to be sure.
You should do the same. Then you really know what the clearance needed is. I put that measurement on the dashboard with a Post It so there was no forgetting.
I found the trailer was 12'8" high or 152". In my case I also worked out the metres as I had to deal with those also.
If you travel the I-5 you should not have any trouble if you are under 13'.
There are truckers atlases which will show clearances of less than the state minimum although they will not tell you the clearance of every overpass.
Most places the state clearance minimum , I BELIEVE, is 14'. That includes California, Oregon, Washington. Of course there are exceptions and that is what these atlases are good for.
I have a Rand McNally , Motor Carriers Road Atlas from ;2007. ALthough it is a few years old now since we are no longer travelling it shows few less than 14 and even those are more than 13'6" unless you get off the main highways into the towns/cities. Even so there are few. I will point out there are places much lower but if you stick to the main AND KNOW YOUR REAL HEIGHT you should be good.
Other states may be very different so get that atlas or similar.
Even so I had a pretty good idea of the feel of clearance to overhead light standards, gas station rooves, and so on to the point I would stop, get out and look.
A last point is if you are travelling big rig routes they will almost always be higher than you. But do the measurement.
