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Web research seems to indicate that most fish do drink plenty of water, some continuously. Some fish absorb water through their skin and/or gills, and may excrete water that way, too. Apparently, fish need to maintain a fairly high concentration of salt, so how a fish deals with this need depends on whether it is a saltwater or freshwater fish. Most saltwater fish get enough or too much salt, and so preventing water loss is their concern. Freshwater fish are short on salt, and since water dilutes sodium, they have many mechanisms for excreting water. It is interesting to note that kidneys process water for excretion, so some saltwater fish have dysfunctional kidneys or are missing kidneys to help prevent water loss. The four "water/salt" strategies are listed below:

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Admin Note: The problem with fish vs. watery environment has do with the process of osmosis, defined as 'net movement of water molecules through a semipermeable membrane from a dilute solution to a more concentrated solution. The 'membrane' is the 'skin' or any other part of the fish that separates it from the water. If the concentration of salts and other solutes in the fish is greater than the outside, watery world, the fish takes on water (natural attempt to 'dilute the fish to what it's like outside' - bad for the fish). In scientific terms the fish is hypertonic to its surroundings (freshwater fish, e.g.). If a fish is hypotonic to its environment (marine fish), then the seawater has a higher solute concentration than the fish; the fish loses water to its surroundings (also bad for the fish). Kidneys function to overcome the effects of osmosis so fishX can stay either more or less concentrated than its environment.
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1. Agnathans - hagfishes

There are basically four different strategies of regulation of internal water and total solute concentrations used by fish. This depends, in part, on the environment in which they live. The first osmoregulatory strategy (control of the excretion of 'osmotic' compounds such as sodium, chloride, urea..) is that used by the hagfishes (Agnatha, Myxiniformes) which are slimy eel-like animals that are found only in deep- water marine habitats. Actually they have a very simple method, their body fluids have basically the same total salt concentration as sea water and they are the only vertebrates with this characteristic (Moyle and Cech, 1982). In other words they are isotonic (equal concentrations) with their environment and there is no osmotic gradient by which fluids or salts can be lost.

2. Elasmobranchs - sharks, skates and rays

The second strategy is that employed by marine elasmobranchs (sharks, skates and rays). Although their body fluid has a lower concentration (hypoosmotic) of salt than sea water (about 1/3 of sea water) they have developed a strategy to overcome this. Instead of passing urea (which is mostly composed of organic salts) out of their bodies, it is put into their blood stream, effectively raising the concentration to that of sea water! Even with this method, they must still eliminate excess sodium (Na+) and chloride (Cl-) ions (Moyle and Cech, 1982). This is performed by a special gland known as the rectal gland, which concentrates Na+ and Cl- ions into a solution that is passed out of the body (Gordon, 1977). The coelacanth also uses this mechanism.

3. Teleosts - freshwater fish

Freshwater fish (teleosts) have the exact opposite problem, their body fluids (1/3 the concentration of sea water) have a greater concentration than their surrounding environment (hyperosmotic). As a result they are constantly taking on water by diffusion through their skin and, to a much larger extent, through the thin membranes of their gills. Therefore, to maintain the high concentration of their body fluids, they must continuously excrete the excess water they have absorbed. This is accomplished by highly efficient kidneys which produce a very dilute urine (Moyle and Cech, 1982). The only problem with such a high rate of urine production is that a loss of salts and other solutes is unavoidable. Salts, mostly Na+ and Cl-, are also lost by diffusion through gill membranes. Some of these can be replaced by ions contained in food but by far the most common method is through the movement of a substance against an osmotic gradient through the use of energy. This usually involves the exchange of one substance for another. In the case of freshwater fish, Na+ ions are taken from the water and ammonia ions are taken from the fish and they are exchanged. This effectively rids the fish of ammonia. Chloride ions are exchanged for carbonate ions which helps in maintaining the pH of the body fluids.

4. Marine teleosts

Marine fish (teleosts) have the exact opposite problem to that encountered by freshwater teleosts. Their body fluids are, again, 1/3 of that of sea water but this time they are in sea water so their body fluids are hypoosmotic to their environment. As a result they will tend to lose water by osmosis to the environment through their skin but mostly through their gills. Consequently, they have developed mechanisms and behaviour to compensate for this water loss. Firstly, the kidneys of marine teleosts are modified in such a way that very little water is extracted from the blood, some species even lack certain kidney structures and can't eliminate water (Gordon, 1977; Moyle and Cech, 1982). This results in a reduction in the loss of water by the production of urine. However, water is still being lost by the gills and this cannot be stopped, so the only method left is to somehow replace the water as quickly as it is lost. Marine teleosts accomplish this by actually drinking water, the most reliable drinking rates reported in the literature range from 3-10 ml/(kg hr) (Gordon, 1977). However, drinking water by itself cannot solve the problem, a complex series of events must first occur in the digestive tract. These events are not yet well understood but it is known that most of the water is absorbed as are the monovalent ions Na+ and Cl- (they are, after all, drinking salt water!), while the divalent ions (such as magnesium and sulfates) are excreted by the kidneys (Gordon, 1977). Sodium (Na+) and chloride (Cl-) also move by diffusion into the body through the gills. Therefore, Na+ and Cl- ions will accumulate in the body of the fish and must be eliminated, this is accomplished by special cells in the gills called chloride cells, which me these ions out of the body by active transport (Moyle and Cech, 1982; Gordon, 1977).

(Thanks to E. Lund - Rummler-Brache Corp. - Investigator - For providing this info) 

อ้างอิง
http://www.madsci.org/posts/archives/1996-12/847855291.Zo.r.html