I'm not sure there would be a way to combine these two excursions. The Bob Marley trip will take at least 5 hours and I don't think you will be in port long enough to plan for both. I haven't done the Marley tour personally, but have friends who have and they raved about it. However, they were huge Bob Marley fans. On the other hand, I think everyone should experience Dunn's River Falls at least once in their lifetime. For what it's worth - JMHO.