Merion told you that children of the village are forbidden from venturing into the Wadewater... which makes it very likely to have been Tegwyn's destination.
'You say the boy might have found Uruk tracks leading north? If he goes that way he is likely to blunder into the Wadewater. Children of the village are forbidden from going there, but I will not seek to tell you that it has prevented the more adventurous sorts from exploring that marsh. If Tegwyn tried tracking the Uruks there... well, there are plenty of dangers in the Wadewater to begin with, let alone our recent troubles!
'You should follow the road north-east to the Dunlending town of Lhan Garan. It sits on the eastern side of the Wadewater, and their scouts may have seen the boy. Perhaps he went to speak with the Dunlending villagers there, and is even now enjoying their hospitality or their tales? We should not leap to judgement too soon; he may be in no danger at all.'
The Dunlending village of Lhan Garan is north-east of Mossward, on the eastern side of the Wadewater. It would be a good place to start looking for Tegwyn.
Avanc in the Wadewater would be a real danger to Tegwyn. Defeating any of them you encounter can only help in your rescue efforts.
Meneldir asked you to listen for Tegwyn.
Meneldir suggested you continue the search for Tegwyn further to the west.
You found a worn wooden sword abandoned in the mud of the Wadewater.
Meneldir asked you to listen for Tegwyn.
You heard a faint voice calling for you to the north-west in the Wadewater. Is it Tegwyn?