What Darwin did was to observe that living creatures have adapted to their environments. He did not accurately explain how this happened in terms of a physical mechanism, because random mutations have not proven to give rise to sufficient sustainable changes. In other words, by what mechanism does a species change from land dwelling to sea creatures? It isn't because one day a land creature happened to randomly give birth to a sea creature and the sea creature by natural selection had more luck breeding and surviving. A line of changes occurred over millions of years by which the species changed in a purposeful direction. When it was done the land species was gone and the sea dwelling species existed. But what process triggers and guides these changes is unknown. Nature seems to have an inate ability to develop life, and life seems to have an inate ability to progress to more complex organisms including consciousness. This did not all just randomly happen from a random single cell of life. It is following a path, whether that path is God or nature.
Ongoing, all the time, yes.
He did not accurately explain how this happened in terms of a physical mechanism, because random mutations have not proven to give rise to sufficient sustainable changes.
Even most creationists accept something they call microevolution. I thus cannot even recognize what you think you're grandly proclaiming here. You are making an unsupported assertion which flies in the face of what most people on both sides of this debate know. I suggest you back it up.
In other words, by what mechanism does a species change from land dwelling to sea creatures?
Random variation and natural selection. When Darwin said that, the only evidence that land dwelling creatures become sea creatures was from comparative anatomy. That is, whales are mammals and sirenians are mammals and pinnipeds are mammals and sea otters are mammals, etc. Whales have almost a fish body plan, sirenians less so, seals less yet, and sea otters still look very terrestrial while spending much of their time in the water.
Darwin predicted evidence of actual transitions would be found. They were. Among the items in that preceding link may be found transitional sequences from land animals to whales. There's also a fossil legged sirenian. Virtually none of the items on that link were known to Darwin. They can all be regarded as successful predictions.
It isn't because one day a land creature happened to randomly give birth to a sea creature ...
It's useless to wave your wilfull ignorance of how a thing happens against evidence that it did and does. You don't know what Darwin said. You don't know what modern science is saying. You don't know what you're talking about.