Could a submarine use a concealed underwater cache to replenish supplies?
Short answer
It is technically possible, but finding and handling a secret underwater cache is harder than meeting a mobile tender hidden in a bay.
Notes
A submarine could theoretically collect supplies from an underwater facility if it could locate the store, connect with it, and recover the cargo. Concealment creates a difficult requirement: friendly submarines must be able to find the supplies while others cannot. Several tenders tucked into bays are offered as an easier alternative. A submarine could find one of those vessels and transfer stores quickly.