We need to know how far you have to walk to get to the "real" beach on Half Moon Bay from the La Sirena condos beach, which is behind a retaining wall. Any info would be very appreciated!

Ok this is two years late, but...
the retaining wall you refer to is actually a sea wall,and man made beach and if you walk about 30 feet to your right, just through the path in the rocks you will be right on a 'real' beach!
Maybe this will help others who want to check out this cool, private condo complex!

For other travelers ...

Half Moon Bay is a big horseshoe bay in Akuaml where most of the hotels and condos are located. Part of the shore is a nice sand beach this is what Pat was referring to as the "real" beach. On other parts the shore is rocky and rough...though this makes makes for better snorkeling.

La Sirena is located on the North side of Half Moon Bay and as Allison mention the beach would be to the right as you face the bay from the complex...though I cannot be certain of the distance to the sandy beach.

Snorkeling is good near La Sirena.