Swim to the part where the drawbridge will lower (between the chains). This is the space between the tower and the broken part of the bridge. Swim down until you see some bones.

Opposite the bones is an alcove...

... that contains a Treasure (2/30) and a Health Potion.

Once you get them, exit the alcove and turn right.

As you swim along the path, keep an eye on the left to spot a Treasure (3/30).

Turn around and swim past the alcove and the bones.

Soon, you will see another Treasure (4/30) lying on the right hand side.