You asked about the flood and where all the water came from and how shells could be found on top of mountains. You are assuming at the time of the flood mountains of the height of Mt. Everest existed. So again lets start from the beginning.

Gen 1:6-13
6 Then God said, "Let there be an expanse in the midst of the waters, and let it separate the waters from the waters." 7 And God made the expanse, and separated the waters which were below the expanse from the waters which were above the expanse; and it was so. 8 And God called the expanse heaven. And there was evening and there was morning, a second day.

9 Then God said, "Let the waters below the heavens be gathered into one place, and let the dry land appear"; and it was so. 10 And God called the dry land earth, and the gathering of the waters He called seas; and God saw that it was good. 11 Then God said, "Let the earth sprout vegetation, plants yielding seed, and fruit trees bearing fruit after their kind, with seed in them, on the earth"; and it was so. 12 And the earth brought forth vegetation, plants yielding seed after their kind, and trees bearing fruit, with seed in them, after their kind; and God saw that it was good. 13 And there was evening and there was morning, a third day. NAS

Science believes that at one point in the history of our planet a canopy of water vapor surrounded the earth, which maintained an even temperate climate required for the growth of dinosaurs. Read verse 7 above. Science also theorizes at one point in the history of the earth all the landmasses were one continent, which science named Pangaea, read verse 9 above. What was the topography of Pangaea, we don’t know. Was the breakup of Pangaea a slow tectonic move of the continental plates or a catastrophic movement that slammed the continental plates into each other? By taking the second scenario we have massive tidal waves that history has shown can reach 300 to 400 ft. in height in comparatively short distances. So with a single continent breaking up the size of tidal waves could reach to even greater heights flooding the entire continent. Now lets look at the Biblical account of the flood.

Gen 7:8-12
8 Of clean animals and animals that are not clean and birds and everything that creeps on the ground, 9 there went into the ark to Noah by twos, male and female, as God had commanded Noah. 10 And it came about after the seven days, that the water of the flood came upon the earth. 11 In the six hundredth year of Noah's life, in the second month, on the seventeenth day of the month, on the same day all the fountains of the great deep burst open, and the floodgates of the sky were opened. 12 And the rain fell upon the earth for forty days and forty nights. NAS

Read verse 11. A volcano or some other natural disaster probably disrupted the water vapor canopy causing the rain and a catastrophic movement of the tectonic plates caused tidal waves bringing seashells and marine life onto the land. This same tidal action could have brought sediment from the sea floors and covered the marine life, which developed into fossils. The Himalayas, Alps, and Andes could be the result of tectonic plates slamming into each other instead of a gradual eons long push. I understand this is merely one scenario out of many possible ones; however, it is one that is viable.

Was there a worldwide flood? I have studied mythology and I discovered an interesting fact. Every culture in the world from Greece to South America has a flood account, in some cultures the ark is a giant turtle, in others only one man survived and used trees, rocks, etc. to magically repopulate the earth, the accounts are many and varied, however, the mere existence of these myths indicates every culture has some reason, other than geological evidence to believe in the flood.

As a reasoning, logical individual you can discount my scenario and mythology, but at least consider what is found outside a test tube with an open mind.