To further discount the eyewitness who claims to have 'seen them' around 7am the same day, we need to go back to the science of the deceased bodies. When they were first spotted, they were floating in the water. You can see that information in this link from October 24th.
2 bodies bound together discovered floating in New York's Hudson River: Police
Add to that they were fully clothed, which would have added to the weight of their bodies.
Now if you want to time how long it takes for a drowning victim to float to the surface, here's a good article describing the biology behind drowning:
http://www.operationtakemehome.org/sar/Fire and Rescue Personnel/Biology of drowning.pdf
"if the water is cold, bacterial action takes place very slowly and it may take several weeks before the body appears on the surface. When a body is fully distended it is almost impossible to sink even with counter weights."
In other words, Rotana and Tala's bodies, taped together, and fully clothed, and taped together, could not have been seen floating in the water the same day they drowned. In fact, assuming the Hudson River is cold water, it would have been at least a week or two before their submerged bodies built up enough bacterial gases to float to the surface.
And what about their credit cards being used for the spending spree? We've heard of people's cards being stolen or used after they were dead. The police haven't confirmed the ladies were spotted anywhere. They only say their cards indicated someone was using them to purchase hotel stays and buying items.
For all we know, Rotana and Tala had been drowned since early October, based on decomposition sciences to account for their floating, fully clothed bodies.