Quandary was not quite what Q had in mind. Quite the opposite it seemed as quaint quantum quarries questioned quarterly returns.  Quintile measurements quick to quibble and quiz his losses were quota quip.

Gads that was tough!  I had typed one out using [m] as this was the last letter per your challenge, and realized after skipping page 2 of this thread the rules must have changed.

OK
Letter: B
Theme: Water 

Namaste...it's universal  [ =