Authors also have a responsibility to reviewers. If you submit premature work, not only will you annoy your colleagues who take the time to referee, but you will gain an unsavory reputation over time. Researchers work in rather small communities and reputations spread. Fill out the refereeing form right after the third reading, while things are still fresh in memory. Unless disallowed, you can return the editor a copy of the paper with your markup, to be returned the author.
Some reviewers make a photocopy of the paper and transcribe their comments clearly on the copy.
You can write the editor a separate, confidential, note if you wish. Many refereeing forms have a specific area for confidential comments for the editor. Do not feel obligated to write things to the editor that you do not share with the authors. In general authors benefit from as much as you can tell them. Readers don't want to read poor pieces, and authors will benefit from revising and publishing quality work.