To represent the total cost \( c \) for a member to visit the water park for \( d \) days, you can use the following equation:
\[ c = 25 + 5d \]
In this equation:
- \( 25 \) is the one-time membership fee.
- \( 5d \) represents the cost of visiting the water park for \( d \) days at the rate of $5 per day.