taken the seine cruise with my friends and we all fell asleep- but at different times - so we were able

I will remind you that you have NOT done this with your present group--it is a wonderful way to introduce paris. And I see you plan to do it. Great.

Don't overdo lunch-- a stop at a café for onion soup or such. Otherwise it is hours--and also the need for a nap because of food!!

Gretchen- I always planned on the river cruise but I thought after dinner would be nice - really never considered it for the first day.
Thanks for the food tip...

"After dinner" in Paris is late which is fine, and particularly in summer. But I don't know how late the boats run. Sunset is late in summer, just FYI--9 or 10

"I still like the idea of walking through the Tuilleries and then a cruise. There is a stop at Place d'la Concorde."

The only boat that has stops is Batobus, making it like a water taxi.
http://www.batobus.com/en.html

If you are talking about a "cruise" type boat [no stops] then you are talking about boat trips like this -
http://vedettesdupontneuf.com/home/

We prefer to do the "cruise" at night; timing it to when the lights come on at Tour Eiffel. Check sunset timetable.

Daytime is nice too, allows you to see more - perhaps better for 1st timers.