The lagoons at Ko Olina on the Leeward side of Oahu are beautiful, calm and safe. Simply put, I love the Ko Olina Lagoons. When I'm visiting Oahu, the first place I go is Lagoon 4, and if someone comes to visit me in Oahu, it is the first place I take them to.
When you walk up the little grass hill (about ten steps) at Lagoon 4 and get that first look at the lagoon, it is everything "Hawaii" is supposed to be. Beautiful ocean stretching out as far as you can see, palm trees swaying with a light breeze, a school of fish visible from the sand, perfect water, and a few, happy people on the sand and in the surf. Even though the parking lot will probably be full (there are very few parking spaces available), the lagoons always seem fairly un-crowded.
I used to feel that the lagoons were somehow second-rate because they are man-made. I used to come here to relax, but go to North Shore for a "real" Hawaii beach experience. The truth is that the only difference being man-made makes is that there are nice amenities (restaurant, restrooms, walking path) near by and that the lagoons are a manicured type of pretty instead of a wild, coral cutting your feet, waves knocking you over and wild pigs in the bushes type of pretty.
My daughter and I go to Ko Olina every single weekend. This is our favorite beach to go to because it is a beach, but it is completely safe. The water is salty and the most beautiful shades of aquamarine. Fish and turtles swim by the rocks. We hear and see waves crash in the distance. All my favorite parts of a beach are at these lagoons with the added benefit that my daughter can run screaming into the water and splash around up to her waist to her heart's delight without my having to worry about an undertow or waves capsizing her little body. The rugged beauty of the natural beaches of Hawaii is replaced here by well-designed, lagoons framed with well-manicured lawn areas. Lava rock barriers let the tide naturally go in and out, but keep the lagoons protected from any kind of waves. The lagoons are in effect the perfect combination of a swimming pool and the ocean.
A picturesque walking trail connects all the lagoons. Many locals take their Holiday and other family portraits along this trail; standing beside palms with black lava rock and glistening blue water in the background. The path starts by the marina next to Lagoon 4 and ends by the awesome shark, fish and ray pools all the way at the end of Lagoon 1. I highly recommend checking out the shark pool- it is really neat. There aren't a huge number of animals, but you get a close look at hammerhead sharks, unicorn fish, and quite a few other interesting species. I never knew sharks were so colorful! They have copper, pink, white and grey all mixed together on their sides. If you eat lunch at the restaurant at lagoon 1, you can get a table overlooking the shark pool.
There are four lagoons at Ko Olina. Hawaii law dictates that there are no private beaches in the state, so the actual coast line does not belong to the Mariott or Ko Olina people, but since they dug the lagoons out behind the coast line, they can enforce their rules on anyone who uses the lagoons. They are required to provide access for locals (and tourists) but the way they keep lagoons 1-3 relatively private for their guests is by providing very few (about 12 per lagoon) parking spaces. Lagoon 4 is the only one that has a decent-sized parking lot. Still, visitors have to get there by 9am on weekends to get a parking spot at lagoon 4. When you take the Ko Olina exit from H1 West, just tell the attendant at the gate "Lagoon!" He or she will wave you through, then just turn right and drive down to the parking lot of your desired lagoon. Lagoon 2 usually has the least people and has a little (expensive) lunch spot, but all the lagoons are breathtakingly beautiful, have decent snorkeling and nice grass areas.