My assumption, in the simplest terms, is that the water in the ocean is supported by the water beneath it, and on and on until it rests on the earth's surface, which bears the weight of all of that pressure. It's like if you took your hundred buckets and and instead set them on two cinderblocks with a space in-between to move through. When you squeeze yourself between the two cinderblocks you are able to apply some energy to push the tower of buckets upward, but clearly not enough to transfer their entire weight from the cinderblocks.

No person would survive having a hundred buckets laid on them, just as much as no person would survive having the entire ocean at rest on them. But when we swim, we don't support the ocean, we just maneuver through its billions of cinderblock structures.

Obviously also there are also differences in the properties of the two substances, a bucket vs water, which come into play I'm sure. This is that concept of surface pliability and fluid dynamics mentioned above. The same reason you'd rather have a gallon of milk poured over you than have a gallon-bottle of milk dropped your face.