Are they original, look like a faucet? If so they should be replaced with a bronze ball-valve or better yet, a seacock. The thru-hulls are 1/2" if original. To replace the old valve with a seacock requires upgrading the thru-hulls to 3/4". That is expensive and a lot of work.
Pic: Ball-valve on 1/2" thru-hull, seacock on 3/4" thru-hull
I'm going to assume that they are stuck open otherwise you would have just closed them. I'll also assume they are ball valves Are you in the water or on the hard? On the hard is easy just remove the valve and Conan them after applying a penetrating oil and letting it soak for a day or so. If it breaks then buy a new one. If not reinstall once you get it freed up. If you are in the water then try disassembling the handle to get better access to the ball. After letting it sit for a day or so try to move it a little. If you can get it to move at all then return it to its original position and move it back. Repeat (sometimes hundreds of times) to free the valve. The movement lets the oil get to the rust so take your time and let the oil work. An artisan rap with a baby sledge to turn the handle can also get the process started. You probably do not want to “beat it open” though as you risk damaging the hull. Alternately you could remove the valve and put a wood plug in the hole till you can replace or repair it.
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