To determine the cost per additional hour of boat rental, we can break down the total cost as follows:
1. The base rate for the first 2 hours is $150.
2. Delia rented the boat for a total of 6 hours, which means she rented it for an additional 4 hours (6 hours - 2 hours = 4 hours).
3. The total cost for the rental was $250.
Now, we can find the cost for the additional 4 hours by subtracting the base rate from the total cost:
\[
\text{Cost for additional hours} = \text{Total cost} - \text{Base rate} = 250 - 150 = 100
\]
Next, we can find the cost per additional hour by dividing the cost for the additional hours by the number of additional hours:
\[
\text{Cost per additional hour} = \frac{\text{Cost for additional hours}}{\text{Number of additional hours}} = \frac{100}{4} = 25
\]
Therefore, each additional hour of boat rental costs **$25**.