Why are LDS missionaries instructed to stay away from bodies of water?

I saw a post recently asserting that LDS missionaries are specifically instructed to stay away from bodies of water and are discouraged from swimming at all. [Does Satan rule water? - Ask Gramps - Q and A about Mormon Doctrine](https://web.archive.org/web/20140419153849/askgramps.org/3597/is-our-world-the-most-wicked-does-satan-rule-water) : >… >As you know, missionaries serve under regulations that would be considered rather severe by those who are not acquainted with the work. >However, as they are called on the Lord’s errand, and their service is so vital to the eternal salvation of our Father’s children, they are encouraged to dedicate all their time and talents to the work of the Lord. >A weekly preparation day is provided, however, so that essential duties not primarily related to the missionary work itself may be accomplished. >This is not primarily a recreation day, although conservative recreational activities are permitted and even encouraged. > >However, not only is swimming prohibited, but also other activities that may be either dangerous or not within the spirit of their calling. >Missionaries should be particularly circumspect about activities “on the waters,” as there is indeed both scriptural and historic precedent for avoiding unnecessary exposure. >… I had never heard anything like this before. Is this true? If it is, is it just applicable while a person is serving as a missionary? What is the basis for this instruction?