If you put the paddle in the sun for a bit, you should be able to see or hear water bubbling at either the handle or the blade joint to figure out which one you need to reseal.

Once you've figure that out, I usually try pouring boiling water (less chance of damaging paddle) over the joint you want to loosen and if it's not stuck too tight then you might be able to twist it undone while its hot.

If its stuck tight and boiling water doesn't work, you can try gently heating it with a heat gun, but be very careful not to overheat it as overheating can damage/destroy the carbon shaft or handle or both.

If its still stuck your only option might be to hang it up to drain very slowly through where the water got in and then reseal it on the outside before taking it back in the water, but you probably don't want to be off the water for that long.