Boats have fuel, water, and holding tanks. The general materials are fiberglass, plastic, and metal.
Fiberglass
Builders mold fiberglass tanks to complex shapes and install them on boats. Fiberglass is an inexpesive choice for diesel, water, and holding tanks. FRP does not rust or corrode. With the use of special resins, fiberglass tanks hold potable fresh water. Fiberglass should not be used for gasoline tanks as ethanol will dissolve the tank. Use fire retardant resins for diesel. Cabo Ricos have fiberglass tanks. Some builders make integral fiberglass tanks.
Plastic
Bladders are thick, flexible, plastic bags for water or holding tanks. They are great for temporary tankage and flexibly fit in odd locations. Bladders tanks may swell and crack with age. They are morpheous shapes in the bilge and not permanant solutions. Hard plastic tanks of polyethelene are acceptable for water and holding tanks. The hard, molded plastic does not corrode or rust. Because of fire safety, plastic fuel tanks are uncommon.
Metal
Aluminum is one of the most common fuel tank materials. Aluminum should not be used as a holding tank because of corrosion. Monel is a metal alloy with superior qualities than any other material for tankage. Monel is expensive and uncommon on production boats. Many Hinkleys and older Pearsons have Monel tanks. Stainless steel is below monel and above aluminum in quality. Hylas uses SS tanks. Stainless is less vulnerable to corrossion than aluminum. Pitting and weld issues are posible. Black iron tanks are posible for fuel tanks. Iron is more prone to rusting than the other metal materials mentioned.
Conclusion
Fuel, water, and holding tanks come under the broad topics: fiberglass, plastic, or metal. Example materials are fiberglass, bladder, molded plastic, aluminum, monel, stainless steel, mild iron.