Completely safe (or insofar, as safe as you can be swimming in water that channels in from the ocean?) They are the man-made coves created in the community of Ko'Olina to form the beaches for the all the hotels and luau property called Paradise Cove. Each of the coves is almost entirely walled in by huge volcanic rock barriers. The water is definitely calmer the closer to the beach that you swim and the level of calmness of waves crashing around the rock barriers depends on the time of year you go - in summer, the overall calmness of the ocean is greater on Oahu than during the winter when the surf is higher in many areas of the island. In March there had been an ocean swell and so the huge waves up at the North Shore made the surfers very happy. In Ko'Olina you could see evidence of the swells if you looked out towards the ocean well past the lava rock barriers. It then DID make the waters in the coves slightly wavier but nothing that couldn't easily be navigated whilst snorkeling.
There is a paved jogging path that winds along the oceanfront in front of all the hotels at Ko'Olina. I can't remember the names of all of the hotels (for sure some Marriotts and other time-shares) but it starts at the Four Seasons to the right of Aulani (they share this cove), and passes around 4 other coves until finally you get to the Ko'Olina Marina. There are people out walking and jogging this pathway from the earliest of hours. It is a really pretty walk. It is easily visible on Google maps.
Access via the oceanfront to the west or right of Aulani and the Four Seasons ends at the edge of the Four Seasons. There IS a small oceanfront path (I don't know for how long it stretches) but it is reserved for use by the guests of that hotel. There are rocky outcroppings in front of this path which are cool to walk on (with proper footware - the rocks are super sharp) in order to see the waves out past the man made coves or to watch fisherman or to see the sunset but there are signs warning about waves coming up and crashing on these areas so you use at your own risk.
So how to access the smaller, first cove in front of Paradise Cove that exists further west from Aulani and the Four Seasons? The best way is via Ali'i Nui Drive which is the main road through Ko'Olina that passes in front of the Paradise Cove Luau property, the Four Seasons, Aulani, etc. There are detailed directions on the Dis here Aulani board as to the best route to take once you get to the Paradise Cove parking lot. I really think that this cove has the best snorkeling because there is a much bigger rocky area in the middle of the cove that allow the fish to play and hide. It is also the more difficult cove to get to but I think that is for the best or the beach there would be even more overrun = less fish and turtle sightings.
You have to remember that ALL beaches in Hawaii are considered public and can be used by anyone. Perhaps there are beaches in other areas of the island which have restricted or privately owned access but it so happens that here there IS public and unrestricted access to all of the cove beaches at Ko'Olina for all of the residents and vacationers at any property. The only caveat is that the beach at Paradise Cove does close down or restrict access when they are using it for their luaus in the late afternoon.
Wow! Sorry that was so long - I hope it was helpful at least?! We've visited the area 3 times now, but if I've made any errors, perhaps a local or more frequent visitor can correct me!
AloooooHA!