The Lake: Deep psychological waters?

[theatresm2002]

No: HPFGUIDX 43574

I've been wondering why only the First Years have to cross the lake 
to get to Hogwarts at the beginning of the term, and eventually 
remembered something that mythologist Joseph Campbell said, referring 
to a mythic character being pulled from the water:
 
"... she had moved out of the rational, conscious sphere into the 
field of compulsions of the unconscious.  That's always what's 
represented in such adventures under water.  The character has 
slipped out of the realm of controlled action into that of 
transpersonal compulsions and events."  (The Power of Myth, chapter 
5 -- titled, appropriately, The Hero's Adventure.)
 
Now, of course the students aren't actually *in* the water -- with 
the exception of the Creevey boy who falls in (GoF).  A closer 
parallel is crossing the Styx, which makes sense given how heavily 
JKR draws on classical sources...
 
But in either case, you're still headed to an "Otherworld" (for lack 
of a better word), some place or thing unknown and potentially 
dangerous.
 
So --assuming you buy into the idea -- why do only the Firsts need to 
do it?  Is it an initiation rite that can only be done once?  I'm 
guessing that once your unconscious mind has been opened to 
this "Otherworld," you don't have to undergo the "trial by water" bit 
again.
 
Any thoughts?  Sound reasonable, or has Amy been drinking too much 
espresso again?
 
Amy   "theatresm2002"