An idle cycle is when a game character or creature, in this case the Sea Creature, stops
moving (whether being controlled by the player or not). It usually happens and enters the
idle state when the character/creature is literally doing nothing.
If the character/creature doesn't have an idle cycle animation it would be frozen in place and not move at all, until a different animation cycle or event was triggered. In some cases more than once idle cycle is created for the character/creature.
An example being, Nintendo's Super Mario game for the Nintendo N64. If the player wasn't playing the game or left the controller alone for long enough, the Mario character would stand still, breathing heavily. Every now and again he would look around himself and if the player didn't touch the controls for a few minutes. Mario would look directly at the player wondering why he wasn't moving and eventually he would lie down/sit and fall asleep on the spot.
An Example - Mario Sleeping in Super Mario 64:
As you can clearly see from the video clip above, Mario sits down to start sleeping (Cycle One), then after a short time switches over to lying down on the ground to continue sleeping (Cycle Two).
Other Examples of Mario's Idle Animation:
(Super Mario - Heavy Breathing Idle Cycle Sequence.)
(Super Mario - Sitting Down and Sleeping Idle Cycle Sequence.)
As the 'Ocarina of Time' video clip shows, having multiple idle cycles can help to make the character(s) seem more alive by limiting the number of repeating actions they do. Thus adding a varying number of cycles to chose from that help to ensure the game/animation on screen doesn't look static or very repetitive for the viewer/player to watch.