There are several Don Guivanni's in the world so keeping them sorted out can be a bit rough. Easiest is to do it by location. Don Guivanni in Gate of Desert Beach has a quest that you need to complete in order to gain access to the rest of the Chapter 2 quests including the quests that Officer Robert in Paradise has. In turn those quests are prerequisites for the Chapter 3 quests from Andrew and so forth.
In other words you really need to do this quest.
The quest itself is excruciatingly simple. He wants an empty potion bottle and a bottle of oasis water. Get the oasis water by drilling, either in Gate of Desert Beach or in Beach Field 5. It is generally easier to dig it up here in Gate of Desert beach because there are fewer other items to dig up, but if you are drilling for Sun Creams to solve Vagabond Eloy's quest you will likely dig up several oasis waters along the way.
Find the empty potion bottle, find the oasis water and talk to Don Guivanni again. You will need at least 2 more oasis waters to solve other quests in Paradise, so go ahead and dig up several.
