I would just like to add to this, that for the duration that SG-1 and Atlantis were running simultaneously, the new episodes themselves ran in parallel. SG-1 season 8 and Atlantis season 1 each released new episodes on the same night (two part season openers not withstanding), back-to-back. And while rare, they will later on make direct references to things happening episode-by-episode in the other.

Immediately after dialing? the water occluded by the Kawoosh would be vaporized. After that, not much. Its pretty explicitly stated in an episode (don't remember which) that the gate's event horizon is used to filter out what does or does not pass through the gate. It effectively limits it to only solid matter with momentum, or energy, to pass through it. That's why you can have differing atmospheres, or even a gate in hard vacuum without any negative effect to the opposite side.