Waterfront lunch/dinner options?
Peter's Cape Cod in Ste-Anne's is pretty good, reasonably priced, has lots of varieties of fish & seafood, and a big waterfront deck... Cons are that it does it not take reservations (as far as I know) so the wait can be long during peak hours. If you go off-peak it'll be much quieter on the deck (for that "peaceful" atmosphere you're looking for) as well as less of a wait.
Edit: I just reread your original post and see that you mentioned a family reunion. I've only been there with 1 or 2 other people and there were no reservation options, but maybe they do take them for large groups - best to call and ask.
Peter's Cape Cod
160, Ste Anne corner Aut 20
West Island, Québec, H9X 1M6
Their *really* poor Web site if you dare to tread: http://www.bar-resto.com/peterscapecod/
You may consider St. Joseph boulevard in Lachine. Its not EXACTLY waterfront (there is a canal across the street, followed by a greenspace, followed by the inlet to the Lachine canal), but it is very pleasant and pretty.
Starting at 17th, there's El Meson, a family run Mexican restaurant (my favorite), then Pasta Andrea a nice Italian restaurant (I eat there ocassionally), then Hentai Sushi and Grill on 18th (I've never tried it), ad Il Fornetto at 19th, an upscale Italian (I've been twice, but for me, a tad too expensive for the food).
Theres a few others further along St. Joseph, but the view of the lake may be obscured.