...what about the nine hundred year old guys in the bible?
I believe that is true and see no problem with it. Just because we don't observe people living close to a thousand years today doesn't mean people couldn't have lived longer in the past.
God says they did, but you seem to leave God out of the Bible.
Adam and Eve were created with perfect bodies, therefore, their DNA was perfect and free from any mutations. God called His creation "very good." Mutations are genetic mess ups, they are mistakes, so God would not have called them "very good". Satan tempted Adam and Eve to disobey God in the Garden of Eden by eating the fruit of the only tree God had told them not to eat from (the Tree of the Knowledge of Good and Evil) and by doing so, they brought sin and death into a perfect world. This is known as the Fall. God had told them not to eat from this tree, for if they did, they would surely die. And, they surely did die, albeit not immediately. Their bodies started the natural process of aging and deteriorating.
"For the wages of sin is death." God showed them grace by killing an animal and making clothes for them. He kicked them out of the Garden to prevent them from eating from the Tree of Life, which was previously not forbidden. This is not because God was being a big meany-head, but because He was being kind. It would be cruel for them to live forever in a fallen, sin-filled world. We can look around at the world today and see that it is broken.
The world became increasingly wicked (I don't think we have any idea how wicked it was, God does not hold back on His description of the old world in Genesis chapter 6). So, God decided to destroy the world and everyone/everything in it, except for righteous Noah and his family and representative pairs from each animal "kind". And He did (I have read quite a bit to the contrary of what you just posted about there not being enough water. That has been proven false and I'll have to dig it up for you).
So, now all humans can be traced back to Noah's three sons.
Mutations began occuring at some point after Adam and would serve as one likely explanation for the shortened life-spans. After the flood, the ages gradually tapered down to what they are today. By the time of Moses in Exodus (just the second book of the Bible), the lifespans were what we would call normal. See what Moses says in Psalm 90:10, "As for the days of our life, they contain seventy years, or if due to strength, eighty years, yet their pride is but labor and sorrow; for soon it is gone and we fly away."
Another explanation for the lifespans is the "Canopy Theory" which you mentioned previously. The pre-Flood world was completely different. The Bible does not give much detail about it, but some folks believe that the hydrologic cycle was different then due to an atmospheric layer of water vapor and that there was no rain on the earth up until the flood. They get this from Genesis 2:5-6, which says, "For the Lord God had not sent rain upon the earth, and there was no man to cultivate the ground. But a mist used to rise from the earth and water the whole surface of the ground." And from Genesis 1:6-8, which says, "Then God said, 'Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.' 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. God called the expanse heaven (sky)." So, this is debated between Bible scholars. Ken Ham, for example, does not believe there was a canopy, while John and Henry Morris do. Those that do propose that the canopy may have provided some health benefits by shielding the sun's harmful rays and in turn, could have played a role in the longer lifespans. The canopy fell apart in the flood, hence the gradual decline in the ages given. Ken Ham believes mutations adequately explain the shortened life spans. Either way, I just say all of that because you said you wanted to know where Creationists are coming from.