The Mormon understanding of Satan/Dominion over waters
Is it true that missionaries are not to ever swim because Satan had dominion over the waters?
I know I was told in the MTC that missionaries were not to ever swim because Satan had dominion over the waters. So what is the actual Church doctrine on this subject?
The connection between missionary policy and the reference to the "destroyer" riding the face of the waters in D&C 61 is a persistent Mormon urban legend. One must consider that LDS missionaries frequently travel by water to reach remote islands. Before the advent of modern air travel, all overseas missionaries were required to travel by ship to Europe, Asia, and other foreign lands. Missionaries, of course, bathe and perform baptisms in water.
The Church has a general policy prohibiting full-time missionaries from swimming. This is simply a safety precaution to prevent drowning or other water related accidents. There are a number of other mission rules that vary depending upon the mission. For example, some missions prohibit missionaries from playing basketball. Rock climbing is usually a prohibited activity. Mission rules are designed to keep missionaries safe by preventing them from participating in high-risk physical activities.
DETAILED QUESTIONS AND ANSWERS
Background of the revelation
The introduction to D&C section 61 provides background:
- On their return trip to Kirtland the Prophet and ten elders had traveled down the Missouri River in canoes. On the third day of the journey many dangers were experienced. Elder William W. Phelps, in daylight vision, saw the destroyer riding in power upon the face of the waters.
The following revelation was then received:
- 13 And now, behold, for your good I gave unto you a commandment concerning these things; and I, the Lord, will reason with you as with men in days of old.
- 14 Behold, I, the Lord, in the beginning blessed the waters; but in the last days, by the mouth of my servant John, I cursed the waters.
- 15 Wherefore, the days will come that no flesh shall be safe upon the waters.
- 16 And it shall be said in days to come that none is able to go up to the land of Zion upon the waters, but he that is upright in heart.
- 17 And, as I, the Lord, in the beginning cursed the land, even so in the last days have I blessed it, in its time, for the use of my saints, that they may partake the fatness thereof.
- 18 And now I give unto you a commandment that what I say unto one I say unto all, that you shall forewarn your brethren concerning these waters, that they come not in journeying upon them, lest their faith fail and they are caught in snares;
- 19 I, the Lord, have decreed, and the destroyer rideth upon the face thereof, and I revoke not the decree.
What waters is the Lord referring to?
Note that the Lord specifies these waters. Joseph Fielding Smith provides some additional clarification:
- These brethren while encamped at McIlwaine's Bend on the Missouri, beheld the power of the destroyer as he rode upon the storm. One of that number saw him in all his fearful majesty, and the Lord revealed to the entire group something of the power of this evil personage. It may seem strange to us, but it is the fact that Satan exercises dominion and has some control over the elements . . . Paul speaks of Satan as the "prince of the power of the air. " (Eph.2:2) The Lord revealed to these brethren some of the power of the adversary of mankind and how he rides upon the storm, as a means of affording them protection. They were commanded to use judgment as they traveled upon these waters, and the saints coming to Zion were instructed to travel by land on their way up to Zion. Moreover, notwithstanding the great power of Satan upon the waters, the Lord still held command and he could protect his people whether on land or by water as they journeyed. 
B.H. Roberts indicates that this refers specifically to the waters of western Missouri:
- After three days upon the river they reached McIlwaine's Bend where they camped for the night, and here an important revelation was given relative to their own movements and also in relation to the "destroyer" that should ride upon those western waters, and the danger thereafter of journeying upon them. Shortly after landing, and before night fell upon the scene, William W. Phelps beheld in open vision the "destroyer" in his most horrible power ride upon the face of the waters. "Others," continues the Prophet in his narrative, "heard the noise but saw not the vision." "Behold there are many dangers upon the waters," said the revelation, "and more especially hereafter; for I, the Lord, have declared in mine anger, many destructions upon the waters; yea, and especially upon these waters [i. e. of western Missouri]. * * * And now, behold, for your good, I give unto you a commandment concerning these things." Then follows instructions to the saints who shall hereafter journey to the land of Zion, not to go upon the river, but by land, "pitching their tents by the way." 
- [note] Joseph Fielding Smith, Church History and Modern Revelation (1947), Vol. 1, p. 207.
- [note] B.H. Roberts, Comprehensive History of the Church, Vol. 1, p. 262.