This is assuming that Angel even knows anything about the human respiratory system to begin with. Why would he bother to learn about it when it hasn't applied to him since 1753, when very little was known about it at all? As Angelus, he wouldn't have cared. Then, he spent most of his time post-curse brooding while cut off from all human society, and he's been living in alleys and sewers since the '70s, which was when CPR first started being taught to the public. He probably just figured that, being dead, he's incapable of restoring life.
And no, Spike couldn't have been drowned. Not fatally, anyway. Angel was at the bottom of the ocean for months. He would have used up the oxygen in that box within hours down there if his lungs actually did anything with it. He also survived a demon dimension with an atmosphere toxic to humans. So lung action for vampires does happen, but seems to be purely mechanical (facilitating speech, sense of smell, smoking, etc). Still, I imagine anyone would find it jarring to have their head forced underwater whether or not drowning is an option. If Spike was still going through the motions of breathing while his head was under, he would have ended up with lots of water in his lungs, and there's no way that feels pleasant.