I was doing a quest one time (forget which one), and there was this area that I accessed that is PERFECT for high alching. It is the quest where you make an underwater suit, and then dive off the docks in Port Khazard and have to swim underwater with the mermaids to retrieve some sort of item.
Godamn my memory sucks...
Anyways, when you are swimming under water, there is an area where you can actually stand upright and walk around. The monsters in that area are non aggressive, and I remember thinking "man this place would be awesome to bot alching or something" and so I brought some alchs down there with me to test it out. It works perfectly
I am almost positive that you would not be able to get random events down there in the ocean, and the chance of somebody coming down there and discovering you if you were auto clicking or something is very slim.