A stereotypical HAL passenger! 
If “nature and the canal” are your priorities, the differences between the six major lines are probably not going to be a big issue for you. Concentrate on price and itinerary. Also single pricing, if you don’t have a travel partner – the “supplement” is typically 100%.
The Six are (roughly top to bottom) HAL, Celebrity, Princess, Royal Caribbean, Carnival, and NCL. They have c. 92% of the market and most of the 8% is more upscale/expensive. The downscale lines were shaken out after 9/11 and that segment of the market now occupies the inside cabins on The Six.
Carnival’s food is better than its rank would suggest. Royal Caribbean is the most family-oriented, has busy-mom food (the passengers don’t care if it’s good, just that they don’t have to cook it), but good (by cruise ship standards) entertainment. HAL has a reputation for comfortable cabins; Princess for good prices on balcony cabins. Celebrity has the newest ships. NCL is infamous for charging extra for everything.
Don’t worry about sanitation; the Coast Guard inspects, issues report cards, and can even shut them down. Passing score is something in the nineties.
If you want to avoid children, choose a line with a strong children’s program – it will keep them on a separate track from the adult passengers.
A willingness to dress up a bit for dinner may lift you up a social bracket or two. One person’s “country club casual” may be another person’s “my husband dresses better than that to go to the hardware store”. Cruise lines haven’t had masquerade balls for decades.
There are lots of review/chat sites devoted to cruising, with one dominant (you won’t have trouble finding it). They are fannish, but good for specific information.
You might want to try a shortie cruise – three or four day Bahamas itineraries are the usual entry level – before you commit to a longer one. If you don’t hate it, you’ll be oriented for the Big One. And! You may get “past passenger” perks for it, if you stick with that line.