I feel much the same as you, Marshdweller, as a fellow non-swimmer. But if you can't swim, you are well advised to treat water of any depth with due caution, so your wariness isn't irrational. I like an occasional dip in an indoor heated swimming pool, and as long as I can put my feet down on the bottom of the pool, I'm ok. One time I got just out of my depth and the slight current of the water seemed to be pushing me further towards the deep end. I began to splutter and splash but my mouth was under the water and I couldn't push myself up. It really didn't help that my mates (swimmers) thought I was just larking about. Somehow I managed to push myself against the current and get to the side of the pool, choking and gasping. They realised then that I needed help. I think the lifeguard was on his tea break!
I am always dreaming about being caught up in flood water, or sometimes running to get away from encroaching water as it is surging along a street. Fortunately not a situation I have ever been caught up in in real life, though of course in the dreams it seems very real.
Funny you should mention your desire to own a narrow boat. In 2000, I went on a narrow boat holiday with a group of friends. I thought it would be relaxing, but it was pretty hard work a lot of the time. I quite enjoyed it, but it didn't help that every night of the holiday I woke up at least once having dreamt that I had misjudged stepping off the boat and had plunged into the murky water of the canal. By the end of the week, I had become completely resigned to the dreams, so it was a case of "oh no, not again!". My worries that it was a premonition have proved unfounded..... so far!