Adam Clarke on Revelation 21:6

It is done - All is determined, and shall be fulfilled in due time. The great drama is finished, and what was intended is now completed; referring to the period alluded to by the angel.

I am Alpha and Omega - See on Rev 1:8 (note).

The fountain of the water of life - See on Joh 4:10 (note), Joh 4:14 (note); Joh 7:37 (note), etc.

The rabbins consider the fountain of the world to come as one of the particular blessings of a future state. In Sanhedrim, Aboth R. Nathan, c. 31, it is said, "He will show them the excellency of the fountain of the future world, that they may accurately see and consider, and say, Wo to us! what good have we lost! and our race is cut off from the face of the earth."