This quest is difficult to do as the 4th person you speak to (in Tumbes) doesn't give you much info to go on. I found an entry in the olous forums about it, if the info there is correct you need to defeat an NPC to finish it off. So I abandoned it after learning this and cannot confirm the last steps.
1. Get quest from adventure mediator in Lima.
2. Talk to Market Keeper
3. Talk to City Official twice.
4. Sail to Tumbes
5. Speak with City Official
6. Sail to Northwest South America landing point
7. Defeat 2x lvl 40 NPC (Prasse Bandits) by Large Rock (Center of map - but possible any one or two bandits) - this will trigger "gathered necessary quest information"
8. Should now be able to use observe and then search the location near Diagonal Rock (North East of Map)