A seacock is simply a valve with a built in flange into which a thru-hull or mushroom head is inserted. The concept is that the flange is through bolted to the hull passing all the way through hull & the backing plate and the bronze bolt heads are then countersunk and faired over with filler on the exterior. A flanged seacock or a flanged adapter, as pictured above, is far stronger and also much safer than screwing a ball valve directly onto the thru-hull or mushroom-head. The cross section of a threaded thru-hull or mushroom-head is very thin. Because of the threads cut into it it becomes easier for it to break off in rough weather. All it takes is a tool box or other large or somewhat heavy item to slam into the valve and snap the thru-hull off to sink your boat.
This is why you should consider using a proper flanged seacock or flanged adapter. I had this happen to me. A spare alternator hit a through hull on our old Catalina and cracked it. While the water did not rush in, and the crack was small, it was a stressful ordeal that required a haul out and repair and is one of the reasons I now only use flanged seacocks or flanged adapters.
The flanged adapter is a relatively new concept in seacock technology from Groco.
In the traditional sense it's not what one thinks of when they think of a proper seacock but it certainly has some benefits over a one piece seacock. I chose to use flanged adapters for a few reasons over a seacock with an integral flange and valve.
First they allow you to replace just the valve portion, in the future, without ever having to replace the flange or worse ripping out an entire one-piece seacock. Second they accept a standard NPS or straight thread thru-hull into the bottom and a standard NPT or tapered thread ball valve threads onto the top thus eliminating the dangerous mismatching of threads.
Groco, however, is not the only game in town and products like Spartan Marine's seacocks have been reported, by many, to last close to 30 years, possibly more, with only the occasional re-build which is quite easy. Many folks are also happy with Marelon which are also resistant to corrosion. Choosing a seacock configuration that allows you to sleep at night is what's most important.
In this photo you can see the flanged adapter screwed to the work bench, the thru-hull on top of the step wrench, the bronze ball valve, a bronze street elbow, teflon tape and some "pipe dope". If you click the photo & blow it up you'll see the difference between a tapered thread (on top of the flange) and a straight thread (the thru-hull). This should give you a better understanding of why a NPT ball valve, like the one pictured, should NEVER be threaded directly onto a NPS thru-hull!!
Hamilton Marine in Portland, ME stocks the Groco IBVF Flanged Adaptors.