We have a timeshare at Ko Olina, and always enjoy our stay. The lagoons are wonderful - although, they are manmade, so don't have the natural beauty of other beaches on Oahu. The lagoons were constructed so that there is constant water flow, though few waves, so the lagoons aren't stagnant water - they are also very safe from sharks due to the constant water flow. A beautiful place for everyone - we love it there!
A note for those who think any of the lagoons are "reserved" for hotel guests - All beaches in the state of Hawaii are *public property* and can be visited by anyone and everyone. None of the lagoons are "reserved" whether they're in front of the Four Seasons or not (although cabanas and umbrellas on the lagoon beaches are reserved for hotel guests/owners). And, so we're clear, "no locals" is not a positive thing to say - its code so creepy prejudiced folks don't have to say what they really mean. In general though, we've found that everyone on the lagoon beaches enjoys themselves with no problems.