Witchers train to handle themselves without relying on their eyes for a lot of reasons. Could be that you're fighting in the depth of a cave, could be illusions, could be you've got blood or dirt or both in your eyes, could be you're unusually poisoned, could be you've taken a particularly bad blow to the head. So this is fine.
He also can't smell, of course, since lindworm ichor overpowers everything, and his sense of touch is a bit dulled by the tingling of long exposure, just enough that trying to feel open spaces by airflow is out, but he can hear fine, and witchers know to always pay attention to the lay of the land and always pay more attention to the lay of a castle.
And his clothes are all still more soaked in poison than is safe, so he might've jumped in the moat anyway. He wouldn't have had to jump off the wall first, but he'd still probably have gone in the moat. It's not even difficult to swim since he had to take off his armor during the game. And by the time his clothes are clean enough to go to Roach, his eyesight will be back to normal.
He hears the thunk as an arrow sinks into the soil of the bank a few feet away. It's fine. It's still night and they can't see him under the water and he can hold his breath for ages. It's fine.