1. **State the problem:** The sea level at low tide is $-2.50$ metres. We want to find the sea level when it is $1.97$ metres above this level. 2. **Formula and explanation:** To find the new sea level, we add the increase to the original level: $$\text{New sea level} = \text{Original sea level} + \text{Increase}$$ 3. **Substitute the values:** $$\text{New sea level} = -2.50 + 1.97$$ 4. **Calculate the sum:** $$-2.50 + 1.97 = -0.53$$ 5. **Interpretation:** The sea level is $0.53$ metres below the reference zero level after the increase. **Final answer:** The sea level is $-0.53$ metres.