I've been researching this too. Island Windjammers and Blount are two casual, small-ship lines that will let you book the regular rate is you're willing to be paired with a roommate. I think both these lines are a bit offbeat - they're really casual, and the cruises are pricey even though they're not luxury. But they look really appealing to me because I want to spend my entire vacation in flip-flops and bringing a prom dress is a minus.

I found this cruise critic article helpful.

Island Windjammers:
Solo Sailors
For those traveling solo, we offer the option of paying the published cruise rate and being paired with a same sex cabin mate. Solo Sailors also have the option of booking a private cabin for a premium “Single Supplement” rate. Single Supplement rates are not valid in Owners’ Suite and are subject to blackout dates. Holiday surcharges and other restrictions may apply. Check out Sagitta's Solo Sojourn, an exclusive sailing for Solo Sailors. Fares on request.

Blount:
Several options are available if you are traveling alone. We do have select cabins configured as single accommodations and priced for single occupancy only. We also offer a “Willing to Share” program that allows you to pay a per person rate for a double occupancy room. You will be waitlisted for a same-gender traveling companion, however should no other person reserve under this program you will be placed alone in that double occupancy cabin. We also have designated double occupancy cabins you may reserve at an additional cost. Please ask your reservationist for details.