Some quick questions
 

(1) Is there any way to get back to the first island after you leave with Diego?

(2) If not, is there anyone on the second island who can repair faction relations?

Thanks in advance.

GO TO the Bar on the 2nd Island, almost every customer is from a different faction, talk to these people about faction relations.

Thanks for the reply. Some comments:

(1) There is no fast travel way to return to the first island. Just a very boring air or boat ride. The two cars that I had reset on the map, so I lost all my items in the trunks. Just great!!! :(

(2) Unlike the guy from the first island, each of the guys in the 2nd island bar will only offer to increase reputation if your current reputation with their faction is negative. When I first talked to those people, I had positive relations with all factions, so I didn't see any such options.

Follow-up question:

I couldn't find anyone to increase my CIA reputation. I currently have negative relations with them. Is there someone who will increase my CIA reputation?