What are the bets cruises for a leisurely cruise on these waters. Thanks in advance!
The Seine hour long cruises are all pretty much the same -- go grab the one most convenient to where you are when you want to do. We have enjoyed the Vedettes Pont Neuf and they have live commentary, but I I were at the Eiffel Tower, I would just have hopped the one there. They are much of a muchness.
Cruising on the St. Martin Canal is like watching paint dry. Go tediously through locks once and it is interesting -- it gets not so interesting after that. Take a walk along the canal instead.
Canauxrama does cruises up and down the canal. We enjoyed ours more than Janet did, apparently. They also do Seine cruises, combinations, and others that might interest you. But for just the Seine in central Paris, I agree with Janet, including a plug for Vedettes du Pont Neuf.
I'd only do the Canal St. Martin cruise if I had done everything else there was to do in Paris or if I wanted to just pass time. It's boring and cold. I've done it a few times because different travel companions have refused to believe me when I say how boring it is. i did not bring a sweater the first time (July of 2011) and lesson learned. I like the Vedettes/O'Chateau mash up cruise which is a champagne tasting while cruising the Seine.