"Amphibious" itself is an adjective meaning "able to live both on land and in water".
If you're looking for a way to make a word related to "amphibious", you could use:
* -ly: to make an adverb, such as amphibiously
* -ity: to create a noun related to the state or quality of being amphibious, such as amphibiousness
However, "amphibious" itself is the most commonly used and understood word for this concept.