I like that water will not hit any electrical components,, Just the batteries
Uh,, They are electrical components.
That said however.. Good plan, only thing I would add is before you wash
Step 1; Mark all POSITIVE wires with red paint, nail polish, tape or heat shrink.
Then disconnect and clean the terminals with a tooth brush and baking soda paste (Add just enough water to make it like tooth paste.. Use an old brush)
Then rinse well and re-connnect.. Then do the rest of the batteries with water as you purposed.
Optional.. Use black paint on the NEGATIVE leads.