I usually set mine to autofocus to infinity, tape the lens and then switch it to manual - but everybody is different and there are others on here that have a lot more experience than me and do things differently. I was recently having a chat with Ray Collins via facebook on this. His reply to my prefocus question was as follows (verbatim):
"Hey Alex, i meant locking the focus settings. Find your distance with either auto focus or manual, then i usually tape it in place (waves breaking on you can make the focussing rings move sometimes) if it is an underwater session usually i prefocus to under a ft, if it is a standard shoot i focus on something 3ft (ish) away. If it is big hollow waves where i might not be able to get so close, i usually try 4-6ft away.
At the end of the day you get a nice image or you dont, 90% of the time I find its my positioning rather than my equipment."
I'd say that Ray's advice could be taken as gospel based on the quality of his work.