"The taps are copper with round handles like on a garden tap."
Those are gate valves, and typically they work
by raising or lowering a flat zinc "gate" up and down internally to control the water flow. They're not a whole lot cheaper than ball valves, which are more reliable. And gate valves are pretty much guaranteed to either fail or jam, leving you unable to shut the water flow.
As Alchemy says proper bronze
seacocks are perhaps best, but they're expensive. Marelon (genuine Marelon, not just "plastic") seacocks arguably will be just as good and less expensive. Bronze (not brass fro the hardware
store!) ball cocks would also do.
You should be able to unscrew the gate valve from the thru-hull itself. Shoot in some PBlaster (it breaks corrosion
, it is not the same as WD40 lube) and then use a big wrench, if you are out of the water. If the thru-hull itself isn't secure...replace that too. You'll find plenty of instructions about how to do it properly on the web, and from the manufacturers. Do it properly, don't try to take shortcuts, and you'll never have to do it again. Or worry about it leaking.