There is nothing inherently wrong with using gate valves on a boat as a sea ****. The problem arises with the quality and design of valve used. What is commonly seen is a brass valve suitable for home use. There are high quality gate valves available in a variety of materials and construction methods. and they'll work just fine properly installed.
Small boat sea cocks serve their purpose well if maintained. Ball valves work easily but sometimes too easily if they're mounted to something that is subject to vibration and movement, like a boat. They can be either closed or opened inadvertently. Most of them commonly found are of no better construction than the lowly gate valve from the hardware store. Being a ball valve does not inherently make it safer...just easier to use.
There are no ships that use the same type sea cocks you'll see on boats; there's no effective way of operating them absent power. Gate valves are used almost exclusively, with the telescoping thread type being most common. Butterfly valves are often used but not for sea cocks. Butterflys are commonly power driven and suffer the same weakness that most, other than gate valves, suffer; a potential inability to close by manpower alone. Those long stems you see on gate valves are what gives them their mechanical advantage. Thus one man can easily open or close a valve that may be serving a 36" diameter pipe.
Here's a site I found buy simply googling gate valves: http://www.velan.com/products/pdfs/v...603-99-web.pdf
Further research will reveal the tremendous variety of suitable gate valves, even for your thru-hulls.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.