Final answer:
The most likely motive for the arson in this case would be to collect insurance money.
Step-by-step explanation:
The most likely motive for the arson in this case would be to collect insurance money. The fact that the yacht owner was complaining about the cost of repairs just hours before the fire erupted suggests that they may have intentionally set the yacht on fire in order to receive an insurance payout to buy a new boat. This motive aligns with the harbormaster's report of a wild party on the yacht, indicating that the owner's son had allowed his friends aboard and they trashed the interior, providing further reason for the owner to want to get rid of the yacht.