Most amphibians lay their eggs in water since

 

Because the eggs generated lack amnion, which renders them susceptible to heat and dryness, most amphibians lay their eggs in water. 
They can avoid this dryness by lying in the water. In addition, unlike bird or reptile eggs, amphibian eggs do not even have a hard protective shell. 
Because the eggs absorb water and inflate to several times their original size by diffusion, most amphibian eggs develop significantly larger after they are laid.