The surname Arzu is a very common Garifuna surname and given the story you've given about your great-grandfather I am pretty certain he was Garifuna. I am a Garifuna-American with roots in Honduras and Belize and the story of Garifuna men joining the Merchant Marine during WWII was very common growing up. Many went to Scotland and England to work during the war.
I know several Arzus, though most I know come directly from Honduras, but the families are honestly all related; Belize received many Garifuna migrants in the 1920s and 1930s due to political violence in Honduras. If you have any questions please feel free to reach out. Thanks.