240v dc connected to 24v dc battery bank
You should use a charge controller circuit and either a control to restrict water flow when batteries are full and overgenerating, or to burn off the unused power in a load bank(Can be used for useful heat if you have need).  You need a charge controller designed for your voltage range and designed to charge the type of battery you're using and designed for the voltage of the bank.  Generally speaking, you want to perform as few power conversions as possible, as each conversion wastes power.  What you want is probably something like

Generator-->MPPT 24V battery charger-->Battery Bank

You haven't stated your battery type, but if they're lead acid, they're relatively very tolerant, but still should be treated well because of cost.  Anything that charges the bank should provide the correct voltage, current and duty cycle for the bank, and anything that draws power from it should have at minimum a current limit and undervoltage protection.  For decent battery life you'll probably want to avoid using the bottom half and top 10th/20th of their capacity when avoidable.  There are many ready made products available, the challenge lies in finding one with suitable ratings.

If you intend/need to build a charging circuit, you should find out what voltage your generator produces under load.  Your components must be rated for the highest voltages they experience plus suitable margins.  But your voltage converter should be designed based on your properties under load.  A smaller difference between input and output voltages at the same frequency allows longer duty cycle, which allows a slower switch for the same losses.  If you chose your switch based on the assumption that you were chopping 200V down to 24V, you would end up with a faster switch than the one you actually needed to chop 30-50V down to 24, not necessarily a bad thing, but also not necessarily a cheap one.  If you choose wisely, it's possible that an off the shelf unit will have features like MPPT that would be very difficult for you to build yourself.  If you build your own your goals should be safety and efficiency and good design practices, and if things go well, maybe learn about microcontrollers and build yourself an MPPT controller.