Here are a couple of ideas (one of which- guess which, I really tried and it
worked):

Thru hulls stick for a couple of basic reasons- calcium fouling (head
discharge), corrosion and sometimes just lack of lubrication.

If the thruhull is stuck closed, then you can remove the hose and fill the
tailpiece with a mild acid solution to dissolve the scale or corrosion
products. CLR a household scale remover (on your grocers shelves) does a
pretty good job. Mix it with water 50/50 and let it sit all day. Then sponge
out the liquid, flush it with a few changes of water and if you are lucky it
will free up. If not then soak it with your favorite penetrating solvent:
WD40, Kroil, etc. If that doesn't work then it is time for a haul out.

You can also remove the drain plug on the side of the valve body (if it has
one) and squirt some solvent in, but the drain plug usually faces down, so
there will be little solvent retained. You could make a fitting to screw on
and fill a tube with solvent to slowly work its way in. Hey if you are
desparate, you can try anything- see below.

If the thruhull is stuck open, then life is a lot more difficult. You could
use a variation on a trick to keep thruhulls from freezing in hard winter
climates while in the water. Find a hard closed cell foam of some sort. Cut
a plug a bit bigger than the ID of the thruhull. Then pull the hose off and
jam the foam plug in the open body of the thruhull and with a pusher of wood
or whatever, push the plug so it is beyond the beginning of the valve, but
not too far to fall out the back side. Now you have access to the thruhull
and you can soak as above. This procedure is not for the faint of heart and
you better have a soft wooden plug handy to close off the flood if the plug
doesn't fit.

Good luck. A haul out may be pretty cheap by comparison with the
consequenses of the above.

David