1) put new block overtop of it
2) dynamite
1) type gm 1 in the console (opens with ~ key next to 1 and below Esc) then press F5 then 5 to select water and right click to remove the small water mound.
2) you need the world edit script to remove it, no other fast way except breaking it one block at a time. or dynamite but you will have to fix the ground afterwards
1. You could throw ground on the water. It works but there could still be water under the surface.
F5 is the best option. 
hmm are you playing on a MP server or on SP? because some servers do not allow full creative mode for all players in which case you would need to ask an admin to remove the water for you.
haha hmm that is weird....in SP it should always work....are you sure you typed gm 1 (as in "gm[space]1" without a / before or anything)?
I myself am an admin on EffenDor (one of the MP servers) and I can assure you that almost all multiplayer servers have the area protection script on which means all your stuff is safe as long as they are inside your area so no need to worry about that 
You can protect areas with area protection script. Then nobody can destroy or remove things. (Only on multiplayer servers).
If you are playing on singleplayer you don' t need to turn pvp off. There is in nobody who can harm you, except bears, tigers, etc. or you open a LAN server. 
