Water itself is never OB automatically. It has to be declared by the TD as OB. By doing so in the case of a lake or creek, the line made by the water marks the outside line for the OB boundary by default, with any disc completely inside the boundaries being OB regardless whether wet or dry. As an aside, a water line is a weak definition for OB. If possible, OB should never be the boundary line between water and land. It should be a paint or stake line or some other permanent way to mark it so that the hole plays the same whether the water level is up or down. It also avoids issues were it's windy and the water line changes with waves, even a little bit.
The TD can change the default ruling for islands by specifically identifying the OB rules in more detail, especially when the target is on the island. Typically, islands defined as IB with or without a target or tee on them, are set up so people can access them without wading thru water. In the case of an island hole with a water filled moat around it, a big exposed rock or dry island inside the moat when the water level is down would still be OB even though the island with the pin is IB.