Question

Why must external fertilization in animals always occur in water?

Answer 1

Fertilization is defined as the joining of two haploid gametes, the spermatozoa, and the oocyte, also known as the egg, to restore the diploid state, form a zygote through the process of egg activation, and begin a series of mitotic divisions that leads to cell differentiation and embryo development.

Complete answer:
External fertilization is a type of reproduction in which the sperm of a male organism fertilizes the egg of a female organism outside of the female's body. Internal fertilization, on the other hand, is when sperm are introduced via insemination and then combine with an egg inside the female organism's body.
Water is used to externally fertilize the female gamete by the male gamete because it allows for a longer period of gamete survival and easier movement of sperm towards the egg. External fertilization occurs when the eggs and sperm used in sexual reproduction meet outside of the organism's body. There has to be a way to get the sperm to the egg in this process.
Because the morphology of sperm in most species is adapted to mobility in watery liquids, many animals' sperm use the water to reach the egg by swimming. Typically, such sperms have one or more flagella, which are streamlined in some species. As a result, external fertilization takes place in water. External fertilization occurs in the cocoon of animals like earthworms. There is no requirement for water to be present.
In this way, external fertilization in animals always occurs in water.

Note:
Internal fertilization is the process of fertilization that takes place inside a person's body. It is mostly found in land animals. A method for introducing male sperms into the female reproductive tract is required for internal fertilization to occur. Transferring sperm is done in a variety of ways by different animals. Copulation is used in mammals, reptiles, and some fish, for example. The cloacal kiss is used in birds to accomplish this.