there is no way to absolutely prove a negative.
however, there is absolutely no evidence that a global flood as described in Genesis occurred, and ample evidence that it did not.
specifically:
- there is no evidence of a worldwide inundation within the last 6000 years
- there is no evidence of a species-wide genetic bottleneck within the last 6000 years
- there is no evidence of a global inundation which submerged all land underwater at the same time EVER in the geologic column
- there IS significant evidence of continuous human habitation and continuous bloodlines extending well earlier than 6000 years ago - pre-flood, pre-genesis.
- there IS substantial evidence of a period of worldwide COASTAL inundation, as sea-levels rose several tens of meters at the end of the last Ice Age, 15,000 to 8,000 years ago - pre-flood, pre-genesis.
- there IS substantial evidence of INLAND flooding, in discrete successive waves, as the icelocked reservoirs of meltwater were catastrophically unleashed as ice-walls failed. Again, well before the "global flood" detailed in genesis. Moreover, the survival of these evidences of pre-genesis floods argues VERY strongly that the genesis-flood could NOT have happened.
that's all off the cuff.
the specialists, including those you have been casually dismissing here of late, will gladly give you detailed information if you demonstrate a will to learn.
[Motive disclaimer: Everyone is wonderful. I have no improper motivation in posting this material.]
There's never been a time when there wasn't a lot of land above water somewhere on the world. We can tell that for sure. There's no other way to interpret things like non-pillow lava, glacial scarring, well-preserved dry-land environments with delicate features such as animal tracks, and so forth. These things happened on land, whether or not they were ever underwater later. Furthermore, many of them--the tracks, raindrop imprints, insect or worm burrows, etc.--record tranquil features of surface life that would have been obliterated during burial in some ultra-violent catastrophe.
There is no place and no time that looks like water, water everywhere. It's not that the picture is totally unchanging. The continents have drifted, collided, separated, collided again, etc. However, there's always been plenty of land sticking up above water.
Thus, we can't find a great global flood anywhere, nor can creationists agree on where it supposedly is. Most of them say that practically the whole geologic column is the great flood, which is sillier than claiming it's in some small stratum somewhere. (If you're saying the flood sediments are the WHOLE thing or even most of it, you have to explain ALL the dry-land features anywhere up and down the column all over the world as somehow having been buried in one and the same flood. However, that's the typical creationist approach.)