How rooted in reality does fantasy have to be? In Deep Embrace all of the action takes place under the ocean. I had one scene which took place in an underwater cave that had stalactites in it. I had them fall on characters during a fight scene. I ended up taking them out after someone pointed out that they would not be in an underwater cave because they need to be in open air to form. Oops! Bad writer!
It never really occured to me to do much research on ocean caves. I have looked at photos and try to use them as a basic example. That's about it for research. I don't like to do too much research. I spent about 3 years researching a series of books. I never got round to writing anything but I wrote pages and pages of background story, character bios, and gathered loads of random pictures that I put in files that I ended up never looking at again.
I can only world build for so long before I get bored and take a swan dive into the act of writing itself. I like to let my stories evolve naturally. I will write an outline and character biographies and that's about it.
Ever since that critique made me realise that I was writing unrealistic sea caves I worry about people not being able to read my stories if the settings are not believable. In the scene I am writing right now the main character Perse has entered the underwater lair of Nyx and Erebus which is a giant crescent moon shaped crevice in the sea floor that is riddled with tunnels which lead deep into the sea crust. The tunnels lead to underground caverns full of air. I KNOW these would never exist in real life so I actually made a point to have Perse mention that. She thinks to herself how these had to have been created because such a thing could never have developed naturally. Now this, boys and girls, is called covering your ass. Hopefully when people read that scene they will not be gripped with the urge to scream 'THAT WOULD NEVER EXIST!' and throw my book across the room.