The minivan-sized submersible Titan, operated by U.S.-based OceanGate Expeditions, began its descent at 8 a.m. (1200 GMT) on Sunday.
The Titan set off with 96 hours of air, according to the company, meaning its oxygen tanks would likely be depleted some time on Thursday morning.
"When you're in the middle of a search-and-rescue case, you always have hope," Coast Guard Captain Jamie Frederick said at a press conference on Wednesday.
Titanic expert Tim Maltin said it would be "almost impossible to effect a sub-to-sub rescue" on the seabed.
The robot could also help hook the sub to a surface ship capable of lifting it, the operator said.
Persons:
Jamie Frederick, Frederick, Atalante, Victor, Hamish Harding, Shahzada Dawood, Suleman, Paul, Henri Nargeolet, Sean Leet, Leet, Tim Maltin, OceanGate's, Steve Gorman, Joseph Ax, Tim McLaughlin, Rami Ayyub, Tyler Clifford, Louise Dalmasso, Daniel Trotta, Brad Brooks, Ariba Shahid, Paul Thomasch, Nick Zieminski, Sandra Maler, Raju Gopalakrishnan
Organizations:
Co, U.S, Expeditions, Atlantic, U.S . Coast Guard, Coast Guard, Guard, U.S . Navy, Stockton Rush, Horizon Maritime Services, Thomson
Locations:
Canada, France, U.S, Cape Cod , Massachusetts, St, John's, Newfoundland, French