Why is maximum GM not automatically the best target for a passenger ship?

Answer: Short uncomfortable rolls

Short uncomfortable rolls ✓ — Right. A larger GM gives stronger initial righting arms, but too much makes a ship stiff: it snaps back quickly and can create uncomfortable accelerations. Passenger ships want enough stability, not maximum stiffness. The counterintuitive design target is a comfortable margin, not the shortest possible rolling period.

Larger comfort margin — Close for safety language, but comfort does not simply increase with GM. A larger GM can increase initial restoring force, yet that same stiffness can make accelerations harsher for passengers. Passenger ships want a margin that satisfies stability rules without turning every small roll into a sharp snap-back.

Slower gentle rolling — No. Slower, gentler rolling is usually associated with a more tender ship, not maximum GM. High GM shortens the natural roll period, so the ship can return quickly and feel abrupt. The surprising tradeoff is that a ship can be more strongly self-righting yet less pleasant to ride.