How do you cook limpets?

Well, in our family you eat them as is. Just use one shell to scoop the limpet out of the other and pop it in your mouth. It has to be terribly fresh though. If the limpets were harvested today then you should be fine. Yesterday? Maybe, if they've been sitting on ice. But they might start to taste fishy.
Oh, but to answer your actually question, I don't think boiling is necessary. Maybe a bit of a steam to soften them first.