fire and water; flooding and fire
Refers to the two elements together, often in the context of disasters or insurance coverage. The second gloss 'flooding and fire' is a common extension in disaster-related contexts.
This insurance covers fire and water damage.
水火の被害に遭った家屋が多く見られた。
Many houses were seen that had suffered fire and flood damage.
complete incompatibility; like water and oil
Compound of 水 (water) and 火 (fire). The reading すいか is the on'yomi of both characters.