Are ocean waves diminished, enhanced or unaffected by underwater obstacles

Waves travel over the ocean surface with columns of water molecules moving in a circular motion, as seen here.

As you can see, when the waves reach shallow ground, the friction of the molecules over the seabed causes it to topple over and break. If in your case there is a tall obstacle under the water instead of the gradual slope, the wave energy will but cut as the obstacle blocks the oncoming motion, causing it to break.

If the obstacle ends and the beach returns to its original slope and the water deepens again, there will be a sort of conflict between the oncoming wave and a returning backwash (with it's loss of energy from travelling over the obstacle, the wave will not travel too fare before it runs out of energy completely, and gravity pulls the swash back.), creating a 'turbulence' at the base of the obstacle, disrupting any waves passing overhead.

In layman's terms, the actual wave will be diminished rapidly, but due to the forced breaking, will appear to be enhanced.