And you'd be wrong. As I commented on another thread, my Sea Otter was rammed on the North Oxford by an oncoming boat who was certainly doing well above 4mph. Despite my best efforts at avoiding him, he kept steering directly at me. The force of the impact nearly knocked me off my feet. When I recovered and peered over the side, I expected a huge dent but there was nothing except a mark on the paintwork.
His shouted explanation for his behaviour appeared to be that canals are V shaped and that he couldn't move to the side. As by the point of impact, we were well out of the centre channel and nearly to the bank, this didn't make much sense to me. In fact I was so surprised, I just stood there gazing after him with my mouth open. All those witty things that I could have said only occurred to me later.
As I watched him go on his way, his wife appeared to be giving him an ear full and he proceeded to ram the bank. I'll be charitable and assume that he had a technical fault with his steering..........
I've also been rammed from the rear whilst moored. That was a hire boat so I accept that was inexperience. Although a 'sorry' would have been nice..... But again only a scratch.
So whatever the Sea Otter did to get the dent that you saw, it must have been VERY spectacular. In general the aluminium alloy used is more 'springy' than steel.
As for the concept that they are unsuited to canals.........well there are a lot of them out there going round the system and I've not seen a dented one yet. I've seen plenty of dents on steel boats though.