How does an aquatic animals nervous system work?

An aquatic animal's nervous system would function pretty much the same as their land-based counterparts; also showing a similar degree of variation and complexity. The size and characteristics depend on the species, but the components in aquatic environment are the same as in the terrestrial: the basic unit of the nervous system is a nerve cell or neuron, which has an electrical resting potential created by an electrical potential difference across the cell membrane caused by ions such as sodium, potassium and calcium. This balance can quickly change or become depolarized as a result of a received stimulus, for example from a neighboring neuron via chemical transmitters across a gap (synapse) between the two adjacent nerve cells. The nerves are connected into neural networks, which, together with a brain, spinal cord and nerve bundles (ganglia) throughout the body, constitute the nervous system.

The degree of complexity varies with the organism's need to transport stimuli around the body. The simplest organisms, (including aquatic ones) don't use or need a nervous system. Notionally if you are unicellular there is little requirement for a nervous system, since ostensibly the purpose of a nerve network would be to propagate a signal between different parts of a large multicellular body. The bigger and more complex the organism and more complicated tasks tasks it needs to perform the more urgent the need for an organized means of transporting stimuli. Yet, some relatively large multicellular aquatic animals have nothing which we might term a nervous system, like the sea sponges (Porifera). Sea anemones have a primitive nervous system but no real brain and no dedicated sensory organs; the nerves facilitate responses to stimuli and other biochemical processes but are not centralized. Some like jellyfish are similarly de-centralized, evidencing a network of nerves and ganglia but again nothing we might call a brain, yet are capable of responding to stimuli, including detection of salinity, co-ordination of movement, and in some, specialized sensory structures and a response to light with simplified eyes (ocelli). Arthropods, like crabs, lobsters, shrimp, etc., have a ladder-like system of nerves on their underside with paired ganglia in each segment, terminating in a rather small brain around the esophagus. Cephalopods, for example octopi, often considered the most intelligent invertebrates, have well-developed senses and large centralized nervous system (brain) and have shown the ability to learn and employ mimicry or insight to solve problems. Aquatic mammals like dolphins have acute eyesight and hearing and large, highly developed brains, and evidence social behavior, co-operation, and significant problem-solving and learning abilities.