Saw this on the UK Talking Pictures channel, (old films & tv) though I'd seen it when they first came out. I find myself in the unusual situation of veering towards the Sir-Oblong-Fitzblob review on this one, as it seemed that some part of the script got omitted somewhere, the episode needed a bit more time and they just didn't have it?
Some of Simenon's books are like this, it takes a while for Maigret to shake the characters around to reveal some weaknesses, and gradually bring the truth into the light. BUT - in this one, he only had limited chances to shake anything, one chap wouldn't speak to anyone, another just smirked (until getting bumped off), and the wealthy 'Lord of the Manor' (actually, of the Canal in this case!), would only reveal tiny snippets of the truth, mixed in with a lot of blustering about the failures of his family, and women in general.
There was indeed some nice location filming, which one reviewer helpfully identifies, but there was no forensic clue, just Maigret trying to get significance from the odd remark that didn't gel.
I also thought the plot failed elsewhere, as there had been a suicide, with a note discovered, later said to have been written by someone else, to 'muddy the waters', but unless I nodded off for a crucial minute or two, that didn't get properly resolved. I'm still a fan of the series, and the star, but this episode was a barge with dodgy steering and a leak?