If the viz is like 3 feet and there is any kind of depth below me (like to even 30-40 feet), I get a little uneasy, but usually am familiar with the site and know what's below. A thousand feet (or a lot less than that) in very clear water I think would bother me, though I have never seen such sites. I may get the feeling that I could possibly fall. I don't mind heights at all on land if no possibility of falling (like top of Empire State Bldg., Sears Tower and back then, Twin Towers). But I don't like being on the bottom step of a shaky ladder.