Suitability and rating
– Last Updated: Mar-14-16 5:20 PM EST –
First, the Discovery canoes are flat-bottomed, and thus targeted to the recreational paddling crowd. Initial stability is good, which is the main thing you'll be concerned with. Boats made to handle well in rough water will be more rounded (or perhaps with more of a "V" shape on the bottom in the case of some brands), and will feel more tippy, but are actually no more likely to flip (there are other details here, but I bet this is enough). The Discovery will be fine for the kind of water you wish to paddle, and it extreme shallows, it will be less likely to run aground than a boat designed for faster speed or rougher water. All in all, it's an okay choice.
As to extra seats and weight capacity, ignore anything Old Town has ever written about weight capacity. You don't want to be paddling a canoe with as much weight on board as Old Town says is okay. Good paddlers who need to carry construction supplies to an island or some such thing would be fine in that situation, but for general paddling, that amount of weight is a bad idea. Old Town uses the "six-inch freeboard" rule for determining weight capacity, and a Discovery weighted into the water so far that there are only six inches of freeboard will not have the stability that you desire, and it will move like an oil tanker. No fun, and depending on conditions, it won't always be safe.
Three normal-sized adults (not "big American-sized" adults) is a pretty good load (a relatively heavy load) in any 16- or 17-foot canoe.
Oh, those molded-in seats offer no advantage at all, except that they help to reinforce the hull, which isn't as stiff as the hull of a canoe of better quality. You can add extra seats, mounting them on hangers attached to the gunwales, but in that boat, one extra seat would be a practical limit, unless the extra passengers are kids. If they are kids, short-legged folding chairs (seats being about 8" high) would be fine.