Hydro-power depends on the flow of water to produce electricity, and is therefore a very secure energy source.
The UK does not need to import fuel to produce electricity by hydropower. We are unlikely to run out of flowing water, but our supply might be interrupted by a prolonged lack of rainfall.
Hydroelectric power stations are fuelled by flowing water. Water does not have to be imported or transported to power stations, and is naturally replenished by rainfall.
A prolonged period of time without rainfall might interrupt supplies of water to UK hydroelectric power stations, but only temporarily. Looked at over the long term, flowing water can be considered a secure source of energy.