Time for an iron ball to sink to the deepest seabed

Roughly how long would it take a cannonball dropped at the surface to reach the bottom of the deepest part of the ocean?

Roughly half an hour to 45 minutes. The Mariana Trench is about $11$ km deep. A dense iron ball quickly reaches terminal velocity in water, where drag $\tfrac12\rho C_d A v^2$ balances its weight minus buoyancy; for a cannonball $10$–$15$ cm across this gives roughly $4$–$6$ m/s. Then $11000/5\approx 2200$ s, a bit under 40 minutes; the terminal-velocity reasoning matters more than the exact figure.