Did the law require officers to join the Royal Naval Reserve?

Were Titanic's officers in the Royal Naval Reserve because a Navigation Act required it?

No. Their reserve membership was common, but it is not described here as an enforced legal requirement.

Titanic's deck officers often held Royal Naval Reserve commissions alongside their Merchant Marine jobs. The suggestion that a Navigation Act forced them to do so is rejected. Their membership is described as common, with no enforced legal requirement. The distinction shows a close personnel link between merchant service and the Royal Navy without turning that connection into a statutory requirement.