I did this last night. I went to take off the hose from the raw water seacock by first releasing the two hose clamps holding it. The hose wouldn't budge. I grabbed the hose to wiggle it off, then realized, the whole seacock was moving. I moved it back and forth and also twisting it as I looked at it. I then thought to myself that it needed to be replaced. The seacock, as all of them on this boat, are mounted on what looks like a 1/2" piece of wood. The wood on most of the seacocks, that I've seen, look to be in fairly good shape. This one, the raw water seacock, the wood is soft and falling apart a bit.
Now, I'm researching parts, procedures and price on new assembly's. Since I know I have to get this one replaced, I may end up replacing others.
I looked at this page that was sent to me, http://www.pbase.com/mainecruising/seacock_primer and this one, http://www.pbase.com/mainecruising/replacing_thruhulls on the same site about replacing them.
In the first link, it shows the "preferred and non-preffered" installations. The triangular one being preferred... all of mine looks like the other one. I'm looking for parts between Jamestown Distributors, Defender, and West Marine to get the better price. For the most part, they carry the same items. I'm also looking at using fiberglass board to use as a backing material instead of wood again and getting a Groco flanged seacock. Plenty more research needed.
I wasn't expecting to work on seacocks any time soon. I was hoping to get the boat in the water soon and do some sailing before doing any major items. Since the raw water seacock looks bad enough to cause the boat to sink, it has moved to the top of list of things to do.
It was suggested to me that the water lift muffler (metal can behind the engine) needs to be replaced. Looking at pictures I've had before, I could see that there were signs of leaking in the past. It was suggested to me that I replace the metal can with a fiberglass one as it should last longer. I'm sure I'll uncover other things to fix or replace soon. I just hope I can get them done to get it in the water and enjoy the sailing part of owning a boat too. :-)
Replacing all the seacocks will turn out to be an expensive project but as long as it's done right, it'll last a very long time.
Here's a short video showing the movement of the seacock.
Update 17 Nov 2012,
I was researching different things to replace the raw water valve/seacock for the engine intake. I'm starting to consider going with a regular round through-hull and flanged seacock. This post I found on a forum makes a good point of growth getting under an external strainer and the difficulty that comes to trying to clean and clear it.
This is the space showing where the strainer was removed.
Update 25 Nov 2012
For Thanksgiving day, I looked over the engine and got it running and was able to get the antifreeze through the engine's water system. This short video shows how it was setup.