I'm in your area, and on my previous boat, I had a frozen seacock as well.
Here are some choices-
1. Haul out.
2. Place drop lights with 75 or 100 watt light bulbs next to the seacocks.
3. Use a wetsuit to jam wooden bungs into the through hulls so you can safely work on shutting the seacocks. (with the expecation that you will immediately haul out if they break while trying to free them).
I was unable to haul out, so I went with option #2. It worked fine during 2 very cold winters where the cove froze completely over. Oh, you can also pour RV anti-freeze into these drains. It will force the seawater out, and leave a "plug" of seawater/anti-freeze mix about at the level of the thru-hull and the seacock. This is a "hope and pray" suggestion, not a gaurantee of success. Just to hedge your bets.
S/V Old Shoes
1973 Pearson 30 #255
Last edited by BubbleheadMd; 12-19-2011 at 11:47 AM.