Go to each cruiseline's web site and look for a link to their employment opportunities section (sometimes you may need to check the site map).
If you can't find anything on the site itself, write to the corporate offices and ask what their hiring procedures are for entertainers. (Many use hiring agencies or established entertainment companies.)
Be careful. There are a lot of questionable ads and web sites claiming to have the inside track to cruiseline employment if you just pay their "reasonable" fee. These tend to rank right up there with the "cheap U.S. army surplus jeeps" offers.