This is the normal expression: "The body was found washed up on the shore." or "The body was washed up on the shore." The first is better to describe the situation when the body was first found. It's not necessary or common to make the sentence active using "the tide" though you could do so. The action of the tide is implied in the passive sentence. "wash up" in this case is a transitive verb. Something must be "washed up", e.g. dead fish, wreckage, rubbish. Have a look at this: http://www.collinsdictionary.com/dictionary/english-cobuild-learners/wash-up?showCookiePolicy=true