Ah, the beauty of a newly varnished teak deck.

On the plus side:
 
Offers real protection for the wood.
 
Wood will not dry out and split
 
 " will not absorb moisture and rot
  
 " is unaffected by dirt and pollution
  
 " will not be stained by spills
  

The not-so-plus side:
 
* Varnishing any wood requires thorough and painstaking
sanding and preparation.
  
* The standard for a ship-shape job is 7 coats with
sanding between each coat.I've often done 12 to 14
to achieve desired results.
  
* Additional coats, with thorough sanding between, are
required to maintain the look.
  
* The humidity must be low or a milky appearance will result,
requiring more sanding to remove the affected coat.
 
Characteristics:
Poems have been written about the beauty of well varnished
teak and I'll leave it to them to do their job on this one.
From a purely practical point of view, if you have the time
to do-it-yourself or the money to have a professional take
care of it for you then varnish will preserve the wood better
than any other method and at the same time be a beautiful
accent to your yacht.