There are several types:
The general study of water is conducted by the hydrologist, and the study itself is called hydrology. Use the link below to the Wikipedia article on hydrology. Surf on over and read just the first paragraph. It will be worth it. And if you feel like it, keep reading. The article is a good one, and the nice folks there post all this stuff for free.
An archaeologist studies artifacts and remains from past cultures.
Marine biologist. Actually, a Marine Biologist is someone who studies marine organisms (plants, oceans, and animals), but someone who studies MAINLY marine mammals is a marine mammalogist.
An archaeologist is a scientist who studies clues from artifacts, structures, and other remains left behind by past societies to learn about their way of life and culture.
The name for someone who studies caves is a speleologist or a spelunker.
An archaeologist is the scientist who discovers and studies ancient artifacts. They use various methods to excavate, analyze, and interpret these artifacts to learn about past human societies and cultures.
A scientist who studies water (H2O) would be a hydrologist or a water chemist.
The name of a scientist that studies sports is called a sports scientist.
A scientist who studies floods is called a hydrologist. Hydrologists study the distribution, movement, and properties of water on Earth.
An arachnologist
A virologist is a scientist who studies viruses.
A scientist who studies animals is called a zoologist.
A scientist who studies radioactivity is called a nuclear physicist or a radiologist.
A scientist who studies animal life is a zoologist. There is no particular name for a scientist that specialises in platypuses.
Hydrologist
The name for this type of scientist is a teuthologist
This answer is a entomologistA scientist who only studies fish is called an ichthyologist.
Parasitologist