Tonight's news showed Fontainebleau and a couple of other chateaux - the grounds are mostly underwater, so these attractions will all be closed until the land dries out. In addition, most highways and toll roads are closed, due to high water. It's unfortunate, but a lot of vacations will be ruined this year.
Though the RER C is not going to Versailles Rive Gauche, you can get to Versailles if you use the bus. There are instructions on the official website.
The underground passageway which leads to the Arc de Triomphe will probably be closed, if it isn't already, and there's no other way to get to the top.
The Louvre will be closed tomorrow and Musee d'Orsay will close off certain areas, while they protect artwork stored on lower levels. I would expect this would continue at other museums, as well - check before you go.
The city is planning to install flood barriers at several Metro stations when it becomes necessary, so these will not be accessible. I would check the ratp.fr website for daily updates.
Some bus routes which run alongside the river might be rerouted while the river remains high. This was the case late this afternoon near Bastille. If you're going to either airport, you should allow much more time than you planned. Traffic is not moving anywhere quickly.
The Seine's current is moving at more than 5 km/minute - city officials are warning people to stay off the lower quais or anywhere there is high water present. If you want to take photos, stay on the bridges.