It's a HUGE leap to get from noticing some features in a rock that are similar to those in rocks you know to have been formed in a marine environment to treating as fact the idea that it had to have been formed in a particular marine environment millions of years ago.
Potentially you are correct. If all we had was the similarity then we hold our theory with much more tentativity. But we have more then that. We have correlating and relationship information of that rock with the rocks around it. With that information our confidence in the theory is increased.
We will never know for sure (in the sense of "proven") and it will always be a theory that it used to be a marine environment. Such an explanation though is valid and is scientific because the data supports the theory and the theory explains ALL of the data. So called "theories" from flood geology often lack in one of those two cases. Either the data does not support the theory or the theory does not explain all the data and is therfore incomplete and inferior scientifically.
The theory of common decent is similar. Just the existance of half bird half dino creatures in the past only gives us a certain degree of confidence in the theory that they are related by common decent. We need the evidence from genetics and the fossil [b]progression[b] to raise our confidence in the theory. Once again, since science is always tentative, we will never be able to prove that birds and dinos had a common ancestor. We just are very confident based on the evidence that our explanation is correct.
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)