I think among common PDF viewers for Mac, Skim would be your best bet, but I don't have a mac and haven't tried it.
However, my actual advice would be different. xdotool is also available for Mac. Rather than using latexmk, I'd use a vim autocmd for vim's CursorMoved and CursorMovedI events which saved your file automatically, and then called a script that recompiled your LaTeX source if it's been edited and if there are no errors, uses xdotool to send the keystroke needed to refresh your PDF/DVI display.
But I believe MuPDF is available for Mac as well, and it's probably the fastest PDF viewer out there. Personally, what I do (on Linux) is use a combination of vim scripts and bash scripts so that my file is always compiling as I type, and the MuPDF preview updated whenever compiling finishes without errors. That way the preview updates as I type. There is a lag, so it's not quite WYSIWYG speed, but it's close enough for me.
You can read my method and download the scripts I use here. They're for Linux, but I think it could probably be adapted for Mac. I don't have access to one, so I can only provide so much help, but I think it would be cool if someone tried to make the necessary changes.
Also, there's probably a way to get things going with latexmk too, which I what I used to do, but it turned out to be slower than my own scripts. The LaTeX box plugin interacts with latexmk, and a Mac user I know likes it a lot, but I have never seen its interface with a previewer.
