Which of the following statement would be the best topic for a research paper that is four to six pages in length? . . my experiences deep-sea fishing in the pacific ocean. proper equipment for scuba diving. how to navigate the ocean. why scuba diving is so popular

The best choice would be the last option "Why scuba diving is so popular".  
It is just enough for four to six pages in length research paper. 

Things you could include in the research: 
• What is scuba diving? 
• What are the benefits we get from scuba diving?
• Factors that make it popular 
• Data gathering (based on scuba divers' experiences) 

You can choose the second and third options but never the first one unless you have experienced deep sea fishing in the Pacific Ocean yourself. 

The second option could be a good choice however, it will only cover 1-2 pages. Proper equipment for scuba diving does not need further research. By simply asking an experienced scuba diver, you're good to go. 

For the third option, "how to navigate the ocean". This could be a good choice, however, it may be too long since you need to know about the first and the second options. It will cover approximately 10-15 pages to complete.