In which places is a cruise a viable travel option for independent travelers?

Cruising from Stockholm to Turku is a cheap way to see the nice archipelagoes of both cities while also making some distance. Especially the barren outer parts is hard to reach without your own boat. This photo (by jje over at flickr) may give you an idea of the sight (if good weather....).

It really depends on the kind of traveler that you are...if you prefer sea or land adventures, or fixed costs up front, rather than variable, or prefixed cuisine/activities, as opposed to adventure... That said a cruise from the Pacific Northwest (Seattle/Vancouver) to Alaska would top my cruise destination list. Depending on the season, witnessing the glaciers from the high seas is very different than glimpsing them from solid land.

Really depends on your definition of "cruise". Here are two things that I have done, that you may be interested in:

1. Alaska State Ferry System from Bellingham, WA to the Southeast AK. This is a 3 day ferry ride along the Inside Passage, a beautiful place. Plentiful whale and porpoise sightings, you are navigating through islands are usually within sight distance of land (== not boring). It is possible to bring your car on board to contiue the trip further in Alaska, or not (ferry connections to elsewhere in SE are available from destination ports). The best part is, you can reserve a cabin (making this more cruise-like), or you are allowed - encouraged, actually - to either set up a tent in the rear of the ship and duct tape it down to the floor, or grab a lawn chair in solarium and throw your sleeping pad and sleeping bag on. The ferry itself is large and has plentiful inside space for hanging out, watching life out the windows, etc. There is a cafe or you can bring your own food and use the microwaves and hot water. As you can imagine, these ferry trips attract an audience of young, adventure-minded and/or cash-strapped people. Everyone is respectful and chill, and there is ZERO glitz associated with cruise ships per-se.
2. In many places (Vietnam's Ha Long Bay, Aegean sea, Mediterranean sea), there are plenty of small-ish ships that take people on for a few days of sail for a very reasonable price. Kayaking, scuba diving and sightseeing is usually included; in warmer climates people are encouraged to sleep on the deck. I did a cruise in the Ha Long bay a couple of years ago. My personal opinion was a bit of "meh", but mainly because the ship had a total of 15 people - 14 couples and myself. Being the third wheel in seven different conversations got old really easily. But the experience was definitely more "adventure" and less "cruise".

Hope this helps!