Yes there is only one Ferry company and they have limited sailing dates. They will put on more trips if demand requires them to do so, so it is worth putting a booking through to them well in advance so that they can build up some clients. Contact us if you would like us to place the booking for you.
Your alternatives are not great I'm afraid. 1) You can either go via air charter from Kariba airport to Vic Falls these are quite costly in the region on $250 per person I think. 2) Drive through Zambia, this is a reasonably good road +/- 8hrs from Kariba to Vic Falls. You do have the addded costs of taking a vehicle into Zambia and then back into Zimbabwe. 3) Drive through Zimbabwe along the Matusadona road. This is a strictly 4x4 road and is quite gruelling, will take probably longer than via Zambia, the road is very bad and should not be done alone. 4) By January there might well be a new scheduled airline that will be offering flights from Harare to Victoria Falls. We are waiting eagerly for the announcement. Can't be relied upon 100% so worth having a back up plan.