The y-intercept of the equation \( y = 36,000 - 900x \) occurs when \( x = 0 \). Substituting \( x = 0 \) into the equation gives:
\[ y = 36,000 - 900(0) = 36,000 \]
This indicates that when the boat is just purchased (0 years after purchase), its value is $36,000.
Therefore, the correct interpretation of the y-intercept in this equation is:
The original value of the boat was $36,000.