Reef

Reefs give some defence to sea units and also make excellent hiding places.

In-depth

Reefs give one terrain star prior to Advance Wars: Days of Ruin and two terrain stars in Advance Wars: Days of Ruin.

In Fog of War, sea units can hide there and are only revealed when an enemy unit moves up to them. Units within the vision range of one of Sonja or Lin's units can see the hidden unit while under the effects of one of their CO Powers. In Days of Ruin, you only need to move a unit past a reef in order to tell if there's a unit hidden inside it.

In Advance Wars 2: Black Hole Rising, Lash's sea units get a 10% attack boost on reefs. This is reduced to 5% in Advance Wars: Dual Strike

Movement costs

Movement type	Movement cost
Infantry	N/A
Mech	N/A
Tread	N/A
Tire A	N/A
Tire B	N/A
Pipe	N/A
Oozium	N/A
Air	1
Sea	2
Transport	2