Your choice of cruise line depends on your budget. The New England voyages are limited to a specific season so they tend to be a bit more expensive (supply and demand at work). We always recommend Princess but other lines operate through NE and Canada so just check the itinerary.
As to ports, you require "Quebec and PEI and at least a couple of ports in Maine". From what we have seen, ports include NYC (NY), Newport (RI), Boston (MA), Bar Harbor (ME), St. Johns (NB), Halifax (Nova Scotia), Charlottetown (PEI). You have to do some homework and pick the itinerary of your choice. Depending on your travel dates, you may not get everything you want, e.g., "at least a couple ports in Maine".
You can select to eat at a table for two once you have picked your voyage; it is part of the reservation process. If you find you are not at a table for two when you get on board, speak to the maitre d'. S/he can usually fix that for you. It is a fairly standard issue that gets resolved on the first nights of a cruise.
As for not dressing up, that's all up to you. Formal nights are requested dressy but you don't have to wear a tuxedo and ball gown. Some lines call it "evening chic". A sparkly top and black pants/skirt work. My husband is disabled so a dinner jacket is extremely uncomfortable for him, he wears a long sleeved shirt and tie if we dine in the MDR on formal nights. What we usually do is head for the informal Horizon Court to dine on at least one formal night. It takes the pressure off packing.
I suggest you go to the MDR on at least one of the formal nights. It is usually worth the effort as the food choices are special and it is something every cruiser should experience at least once before saying that getting too dressed up isn't something they enjoy. Try it, you might like it :)
Best of luck