NIV version of the Bible, Genesis 6:14-16
"So make yourself an ark of cypress wood; make rooms in it and coat it with pitch inside and out. This is how you are to build it; The ark is to be 450 feet long, 75 feet wide and 45 feet high. Make a roof for it and finish the ark to within 18 inches of the top. Put a door in the side of the ark and make lower, middle, and upper decks."
Unfortunately our children are taught that Noah's Ark was just a cheesy overgrown boat with it being overflown with giraffes and dinosaurs sticking out over the railings. But when you really examine the Scriptures you can see that God gave some fairly detailed instructions for a seaworthy boat. We do not fully understand all the terms used for the directions but we can get a rough idea.
Take a look at http://www.creationontheweb.com/content/view/1773/ . It explains in GREAT detail the seaworthiness of Noah's ark.
Not all things have physical, natural explanations - the parting of the sea is one of them. Being God, we cannot explain everything in terms of molecular bonds, reactions, etc. Supernatural events are free from the bonds of scientific law and as such often do not make sense.
But I feel confident that Noah's ark can be explained through natural means. While God could have easily created a box shaped boat that, while normally unable to float, could have floated because of supernatural means. However, the dimensions given in the Bible clearly give shape to that of a real seaworthy ship.