Seine river cruise?

Definitely do it at night. The lights are beautiful. You'll be looking up at Paris from the river, so daylight doesn't add much.

At night, for the reasons given by Doug.

Do it at night. A very special memory for us from our Paris trip last summer is our Seine cruise at sunset. The Eiffel Tower sparkles at the top of each hour, and it is lovely to see it from the water.

Night, definitely. You can get just the tour and it's inexpensive. March 2010 it was 11 euro, or thereabouts for the cruise we took.

Another vote for night. The one time we did it at night, it was pouring rain. We still sat on top under our umbrellas and watched the lights of Paris as they went by. They were romantically blurred with the rain. The previous poster is correct, the Eiffel Tower light show is lovely as viewed from the river (it's pretty lovely in general). Definitely no meal or drink. Spend your money on a much better dinner on dry land when done with the cruise (or before hand).

Thank you everyone !
Night it is !!! :o)

and definitely skip the meal option - you will get better food for less money on shore my favorite time is just as sunset, finishing after dark