Any oil that does wash up is likely to be in the form of tarballs which are a "little easier to manage", Rear Adm Peter Neffenger said in testimony.
Reports from the White House say a presidential commission will be set up to investigate the disaster.
And a top official who oversaw offshore drilling has announced he is retiring.
Chris Oynes said he would step down at the end of this month.
There was no official comment on the reasons for the resignation of Mr Oynes from the Minerals Management Service (MMS). Senior officials there have been accused of becoming too close to the industry.
Oil has been spewing into the Gulf since BP's Deepwater Horizon rig exploded on 20 April.
US President Barack Obama has described it as a "potentially unprecedented" environmental disaster.
An unnamed Obama administration official told the Associated Press news agency that President Barack Obama would establish a commission by executive order.
It would, the official said, be similar to panels created to investigate the space shuttle Challenger disaster and the nuclear accident at Three Mile Island.
It would study oil industry practices, rig safety, regulation and governmental oversight, as well as the "structure and functions" of the MMS, an official told Reuters news agency.
At the Senate Committee on Homeland Security and Governmental Affairs, Rear Adm Peter Neffenger said that the government was closely watching whether the oil would be swept up into the "loop current" that moves around Florida.
"Currently it shows to be somewhere in the neighborhood of 40-50 miles [65km-80km] from the southern edge of the spill," he said.
"We are watching that carefully and as a result of that we are preparing for potential impact on the southern Florida coast and impacts around the southern Florida coast."
“ I think we ought to be cautious in terms of defining what plumes are out there and how they're behaving ”
Lamar McKay BP's America president
Tarballs would, he suggested, be "a more manageable piece" than the slick in the Gulf of Mexico.
Homeland Security Secretary Janet Napolitano also appeared at Monday's hearing, the first time senators were able to question officials about the response to the 20 April disaster.
She said the government was monitoring the loop current very closely and were treating it as if it were a coastline.
"In other words... if we were to see that the oil really was beginning to move toward the loop current we would begin doing some things in the way of dispersant and booming... as if the loop current itself were a piece of the coast," she said.
Senator Joseph Lieberman, who chairs the committee, said at the hearing that the government should not allow any new deep water wells to get permits or be drilled until the industry could prove it can prevent another failure of drilling equipment in deep waters or contain oil spills more effectively.
"I say that with regret because I know how important offshore American oil is to our nation's energy independence," he said.
"But the US government has a responsibility to the public safety that is more important and that responsibility, I fear, was not fulfilled in this case."