From the system description, it sounded to me like the hydro power is always electrically connected and they manually open the water valve to start generating power (up to 6A but usually run the flow to generate about 2A) and vice versa. If the water valve were to get stuck in the on position from the failure in an automated water valve, it wouldn't destroy the hydro unless the load (i.e. the battery) were also electrically disconnected, which they don't ever need to do except when they don't want the voltage produced by the hydro to flake out the solar panel charge controller (i.e. when the sun comes out and they want to gobble up as much of those solar rays as possible). Rather than consider disconnecting the electrical power of the hydro (to remove this charge voltage), consider to simply automate turning off the water to the hydro to both stop producing this charge voltage (on the battery from the hydro) and to stop the water flow on the hydro (to save the bearing life) at the same time. That way the battery voltage can drop (with no charging source), be identified by the solar charge controller, and allow the solar charger to work alone. This solution doesn't make the multiple power source work together (15A solar and 2A hydro), but it does keep them from fighting with each other (15A solar or 2A hydro) without having to spend your days hanging out around the water valve looking at the clouds.
As an aside, I saw that you could trickle charge a 500Ah lead-acid battery with up to 5A continuously (much higher than the typical 2A from your hydro) to maintain the battery at a fully charged state without overcharging (i.e. 5A is a low enough charge rate for the recombination reaction).