There are a number of different types of (hydro)gel materials used in horticulture. The most common are super-absorbers such as polyacrylamide, the sodium and potassium salts of polyacrylic and polymethacrylic acids, also starch and cellulose based products. I believe that it is the polyacrylic and polyacrylamide type gels that are used in babies' nappies and other sanitary products.

Once swollen these gels can be awkward to handle and messy. They tend not to like cycling between the wet and dry states. When mixed through composts they can pull water away from plants and may clog up the soil. The starch and cellulose type gels will degrade safely in relatively short times. The acrylamide type gels are generally very stable and do not readily degrade. The building block acrylamide monomer units are fairly nasty.

The term hydrogel is generic and there are many, many different types with varied properties. The super-absorber type of gels tend to hold 95% by weight of water and are often mechanically weak and "sticky" or "slimey" when swollen. These gels are usually just very high molecular weight water-soluble polymers that have been lightly crosslinked to form an insoluble network. Sometimes they are not even cross-linked and the gel is a very viscous solution.

Other hydrogels can be carefully formulated to hold controlled amounts of water when swollen e.g. soft contact lenses (typically based on crosslinked poly(hydroxyethylmethacrylate)).

The hydrogels that I developed for my Osmogro membrane osmosis plant watering system are polyurethanes based on poly(ethylene oxide). I can formulate materials to hold between 10% and 90%+ water. These materials are generally mechanically strong and stable.

I've worked for more than 20 years developing hydrogels for many different applications including controlled delivery of active agents, medical devices, biomaterials, membrane separation processes, sensors and plant watering.
Chris