Why is the word "Ark" used for Noah's boat and the Ark of the Covenant? What exactly is the meaning of an "ark" that applies to a boat and that thing that melts off the Nazi's faces in the first Indiana Jones movie?
We need to be careful thinking that our English (or other language) translations always reflect God's Words perfectly.  The word 'Ark' when referring to Noah's ark or the ark of bullrushes that Moses was placed into is the hebrew word 'teebah'.  The word used for the Ark of the Covenant is 'arown'.  I believe that there are similarities between the two as noted in other answers, but the words given are distinct in the Hebrew.