I haven't found a "standard height" anywhere but on Destiny the rails were 3.5 to 4 feet high. I'm 4'11" and there was no way I could accidentally fall overboard.
Life is too short to let the ship of your dreams sail without you.
Carnival Destiny Feb. 2006
Carnival Fascination Feb. 2007
Royal Caribbean Grandeur of the Seas Feb. 2008 The DTW & MsBJ tour
Carnival Valor MsBJ and Dina Feb. 2009
Carnival Glory MsBJ and Dina Feb. 2010
Carnival Dream MsBJ and Dina Feb. 2011
I'm 6' 2" The rails are about 4 foot high. I do lean over them alot, looking down at the water. Sometimes, you can see dolphins swimming next to the ship. I can also judge the wave height by how the bow cuts the wave .
Under normal circumstances, there's no way a person can fall off a ship with out some added help. These ships are really safe actually.
Dream - Sept. 22 2012
Truimph - Nov. 2010
Truimph - Aug. 2010
Legend May 2009
Liberty - Aug.23 2008
Fantasy - Sept.22 2007
Fantasy - Sept.27 2007 (B 2 B )
Holiday- Sept.- 2006
Valor- May - 2006
Conquest- August- 2003
Enchanted Isle- Sept.- 1999
Although, I always worry about children climbing on the rails and falling over board. Once while aboard a ship in Tampa Bay there was a drunk guy who was not intending to kill himself, just scare his girlfriend. He fell off the ship and by the time the Coast Guard located him. He had hypothermia so bad he died.