Well presumably the purchase cost is based upon the business being a going concern so profits should accrue over the period anyway. Assuming 10% ROI, the profit would offset the interest costs of berthholders since the profit could be distributed by way of dividend after running costs and necessary expenditure.
Don't forget: you earn more than just the berth holder fees. What about the mooring fees from visiting yachts ? I think these cover for the running costs.