As a Parisian, I'm a bit torn by the idea of an opening ceremony on the Seine.
Of course it's gonna be a security and logistics nightmare, but that doesn't worry me too much. We're kinda used to that, every year on Bastille Day, 300k people attend the military parade on the Champs-Elysées, 500k spectators watch the fireworks on the Champ de Mars in the evening, and a few days later, another 300k people attend the last stage of the Tour de France, without any major inconvenience. Now of course, the Olympics are an even much higher scale, but as we say, impossible n'est pas français.
As someone said a few weeks ago on the Tokyo opening ceremony topic, ceremonies are mode for TV, and this one will look amazing if you're watching on TV. The aerial views of Paris at sunset are stunning, and the fireworks on the Eiffel tower always look impressive, so it should definitely be one of the most picturesque opening ceremony ever.
But as incredible as it would look, I'd prefer if the ceremony was held at the Stade de France because I intend to buy tickets for it, and I feel like a ceremony on the Seine would ruin the rythm and the surprises like SeriousPotato said, and kill the unity of space and time that you have in a stadium.