The costs are just too high unless you only seek the romanticism of taking a boat with likely non-English speaking crew for weeks at a time across an ocean. A round-the-world ticket (www.oneworld.com to start) is usually a far better value in terms of seeing many places on a (much more) flexible schedule.
I regret my oneworld ticket approach to a 6.5 month backpack break - in retrospect I should have seen far fewer places and got a better feel for the ones I was in. It's also not nice having flights booked so far in advance that it's hard (not impossible) to stick around in the various paradises you come across.
Actually though you have a ton of flexibility on those tickets. If you need to move a date, it's easy enough to switch to any flight with availability in that class, no charge. Also, you can split it up and take half the trip, fly home for a while, then return and continue the trip where you left off. As long as you fly within a year, you can do segments 11 months apart really.
