If you're looking for alternatives to a cruise ship you might try a tramp steamer. In days gone by this was an inexpensive way to take an ocean cruise. The modern cruise ship with endless buffets,casinos and never in ending entertainment has put this type of travel in a decline.
If you would like to experience a different type of cruise you might want to try cruising on a freighter. Of course you will not have the amenities that are found on a cruise ship. But you might enjoy seeing the everyday life on a working ship. You will have your own cabin but your meals will be taken with the crew. You will visit different ports of call that you may not experience on a traditional cruise ship. You will definitely have time to reflect on your life or read that book that you never had time to do. So if you're looking for an inexpensive way to travel the world this would be for you.