There is a version that makes spc works with smart movement mod.
(See related link below for more information.)
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you can install Single Player Commands and there is a freecam option.
In multiplayer, commands are generally started with a "/" Single player commands have not yet been added, unless you get a mod that has commands, like WorldEdit. They will be added in the next update, 1.3.
Try a mod. You could use InvEdit or SPC (Single Player Commands).
Get single player commands then type in spawn silver fish. Make sure it's not on peaceful.
Each server as its own set of commands. There is not a list of all commands available as they are different from server to server. If you want to know the commands you can use on a single player world, just use /help you the list.