So yes you are super close to being finished there. You just need to delete the 2 faces on the inside that I showed above, then you can select everything and do Edit > Join and have a solid so it's ready to be sent for printing.

If the object type indicator here shows says it's a "surface" or "joined srf" then that means there is some opening in it that still needs to be closed off for it to be watertight. If it says solid then there aren't any naked edges so it's watertight:



- Michael