I believe a seacock has a flange built in that normally gets bolted to the hull. Most brass seacocks also have a small plug (typically 1/8" or 1/4") in them to allow you to winterize, drain lines, etc.
An inline ball valve is just that, typically has female threads on both ends. Most of the time there are some sort of adapters on both ends (brass, bronze, stainless and plastic).
Hope this sense.