They can't be all that bad, there was a welsh fellow hat went across the channel in one! Jim Bridgers' bull boat was basically a coracle too, he didn't seem to do too bad in it!
Yup - the same guy who makes them and taught me to paddle one (and taught me to play badminton...our clubs first net was a "recycled salmon net...)!
He lives just down the road from me....
I wrote up an article for SWAT mag (years ago now) on how to build a Bull boat and examples of coracles from around the world (including a Bull Boat) are to be found in the Coracle Museum in Cenarth on the Teifi River in Carmarthenshire.
The reason that the Bull Boat used by the Mandan Indians is like a corracle is that they are decended from Prince Llewellyns men - who "discovered" America before those pesky Vikings or Spanish/Italians
The Mandans also spoke Welsh - but were wiped out by disease brought in by those darned Sais, deliberately targeting the poor old Welsh yet again!