Why did Touma focus on the fact that you can't bring another person on the surfboard?
They never mentioned the speed limit of the yatch. Is it not possible that both went on the yatch for 15 km in 40 mins? Then the husband killed the wife and threw her to the sea. Then he used the surfboard to go back 10 km in 20 mins and pretended he was pushed off board by his wife.
