Did aquatic life forms get fossilised ? If, yes where do we come across such fossils ?

Yes, aquatic life forms get fossilised in the sediments of the water bodies. Infact there are more aquatic than terrestrial fossil organisms.

Such fossils of sea creatures are found in the mountains as opposed to deep sea beds (in sedimentary rocks).

Earlier they were found at the bottom of the ocean which in due course of time, these oceans sediments were pushed up to form mountains.