Quickly searching, US had more than 8,000 ships during the war. Other site lists number of destroyers in Dec -41 as 171 and subs 112. -44 367 DDs and 230 SS. Not counting frigates, transport ships and auxilary, that is way too many ships to individually assign and organize in fleets, even if the numbers are somewhat off. So I don't see 1:1 modeling happening with HoI3 fleet management.
However I could see it working if DDs and subs would be assigned to flotillas from production lines/storage. And possibility to adjust the flotilla size. Default flotilla size, say 7, could be used in fleets but custom flotillas could be used to escort a single capital vessel or convoy raid in less important areas. Though keeping track of each individual ship would be pain.
Those interested in translating historical ship numbers to HOI3 might be interested in this thread.