This needs some investigation. Discuss ideas and findings on the talk page. Specifically: See if the editor still exists and if it's able to be modded back in.
Decompiling the game's files reveals one with instructions for a level editor. Unfortunately there is no known way to access the editor.
EDITOR KEYS - OLD
TOGGLE EDITING MODE: Press 'E'
LEVEL LIMIT BUBBLE: Hold SHIFT and...
1,2,3 ... 9 - change positionof level bubble to show limit of objects
0 - turn off level bubble (also shows level 10, where all objects are shown)
NEW OBJECT: Hold down SHIFT and...
'K' - krusty krab
'N' - krab patty
'J' - jellyfish (note the height you add the jelly becomes that jelly's max height it can travel to)
'C' + 1,2,3,4,5,6,7 - different pieces of coral
'W' + 1,5,3,4 - different pieces of seaweed
'P' - coral platform
'H' - health bubble
'I' - invincible bubble
'S' - speed (or turbo) bubble
'T' - time bubble
EDIT OBJECT POSITION: First click on the object with the mouse to select it, then...
'G' - increases the tweak by 10 units
'R' and '<' or '>' - tweak rotation
'X' and '<' or '>' - tweak X position
'Y' and '<' or '>' - tweak Y position
'Z' and '<' or '>' - tweak Z position
'S' and '<' or '>' - tweak scale
EDIT OBJECT MOTION: First click on the object with the mouse to select it, hold down 'M" and then...
'G' - increases the tweak by 10 units
'X' and '<' or '>' - tweak X movement
'Y' and '<' or '>' - tweak Y movement
'Z' and '<' or '>' - tweak Z movement
'C' and '<' or '>' - tweak number of cycles (100 is default). More cycles = more movement
'P' and '<' or '>' - tweak phase offset. e.g. if phase = 50 and cycles = 100, then starts movement halfway along