Thanks for all the comments everyone.
250kts: I don’t know the area that well actually; the course of the river in the estuary changes frequently and my GPS track is over a year old.
Bigmuz7: You’re absolutely right. An expanse of water with no points of reference is very unnerving and difficult to navigate. This was the main problem on the day.
Paul: I run aground in this area all the time; about half my trips end with a crash of some sort. Still, you’ve got to crack a few eggs to make an omelette (quick Mere Poulard joke there).
Simon: That’s a very kind offer; I’ll certainly drop you a line next time I’m going to Avranches (in the autumn, probably). Would you like to join me on a trip sometime? And yes, I know about the travellers, they were there the first time I went up the Sée. I’m sure they’re as good as gold, but all the same I’m glad the boat’s back home.
Erin: my cousin was with me and saved the day. Poor sod, he’s just had a serious operation and wasn’t feeling that great. Still, I’m sure there’s nothing better for one’s constitution that wading chest deep in cold, rat-and-coypu-invested water at 6am.