Cryptozoology is the study of creatures whose existence have not been proved, such as the Loch Ness Monster or Sasquatch. Many cultures have myths of strange creatures inhabiting local forests or bodies of water, and in recent times scientists, amateur and professional, have begun to examine these tales and trying to prove or disprove the existence of these creatures.
So, is cryptozoology a valid science? It depends on how you define science. Cryptozoology is based on folklore and eyewitness accounts from people who have seen animals unknown to science. That means that cryptozoological evidence is not as reliable as zoological evidence, as zoology deals with known, scientifically proven creatures. However, cryptozoology has had success in gaining information about creatures such as the megamouth shark, discovered in 1976.
Another example of creatures unknown to science being discovered is the mountain gorilla. The mountain gorilla was discovered in 1902 by Captain Robert von Beringe, a German explorer, and has since resulted in valuable studies. This ape was previously unknown to science because the area had not been explored exhaustively. Much of our planet has still not been explored, especially in the oceans and other large bodies of water, where it is hard to search a large area at once. There could easily be many undiscovered creatures there.
Opponents of cryptozoology often poke fun at cyptozoologists for taking reports of creatures like Sasquatch and Champ (a serpentine creature supposed to occupy Lake Champlain in New England) seriously, but if such creatures existed, then it would help to expand our knowledge of the world if we knew that they existed. Science does not automatically reject things it deems "ridiculous", but tests them or explores them to discover the truth.
Is cryptozoology science? Not if by the word science you mean something easily verifiable or something in your textbook. But if by science you mean the study of nature and the world we live in, then yes. If by science you mean "a system of knowledge concerned with the physical world and its phenomena", as the Merriam-Webster online dictionary says, then cryptozoology is a science. It is a science just as zoology is a science, even though it is concerned with creatures that have not been scientifically proven yet. One never knows when cryptozoologists will discover another previously unknown animal.
In short: yes.