In a letter to Lake Kariba boat operators, dated 29 October 2018, Zimbabwe's Parks & Wildlife Managment Authority (ZimParks) announced that payment for all "goods and services" (e.g. lake entry fees, Park entry fees, fishing permits etc) offered by the Authority must be paid in foreign currency by visitors who are not Zimbabweans.
They further advised that all visitors will be required to provide passport ID for verification of their origin, at ZimParks revenue collection points at harbours in Kariba (e.g. Marineland Harbour, Chawara Harbour and elsewhere) before being allowed to depart onto the lake.
To facilitate this, ZimParks have have opened a foreign currency account in Chinhoyi to receive such payments from operators and booking agents (see details below). Presumably independent foreign travellers will be required to pay in foreign currency cash.
Here is a copy of the relevant letter:-
Wild Zambezi will be monitoring the situation with regard to ZimParks requirements for foreign currency payments elsewhere in Zimbabwe and in the Zambezi Valley's National Park areas in particular, and will report accordingly.