OK I'll have a go.

I was in Cartegena back in 2000. October - it was out of season I think. You could ask in the hostals about taking a boat out to one of the Rosario islands to stay on the beach with a fisherman's family, away from any tourism or commercialisation.
The fisherman provides a hammock (I had my own), a cover if necessary and a mosquito net if necessary.
The family will feed you for as long as you are there.
Everything else you take with you, including drinking water. You can stay there until your water runs out. You can take a long walk, a few hours I think, to hotels for more water but need to carry it back across fields and sand.

The trip from Cartegena is by small motorboat that stops at other places on the way.
You pay a boat fare, a small amount per night, and the price of each meal (3 per day if you want).
Unfortunately don't remember the name of the place but I may have read about it in Rough Guide or similar.

On my visit, 2 or 3 European students were already there. They stayed a couple of days more and left, then there was only me on my own.

When the family were fishing or whatever, I was completely alone. Also from dusk to dawn when they went to bed.
For me it was great but not everyone's cup of tea. The beach was wonderful. Lots of phosphorescence in the water at night. Most nights there was a terrific electrical storm high in the clouds, lightning all over the place. I stayed about 5 days, wished I'd brought more water.
Some days a farmer brought cattle onto the beach for grazing along its edges.

If you have good Spanish (I had none) I'm sure you could negotiate with 'visitors' like the farmer or friends of the fisherman who visited, to bring more water.

It was one of many highlights of S. America.

Another was Margarita Island, Venezuela. I stayed in Juan Griego village on the NE coast, away from all the tourism (at least it was in 2000).

From memory and a look at Google Earth I stayed at Posada Caney, owned by a Canadian married to a local woman.

The town was another great place to stay, with an intriguing atmosphere. All the businesses, cafes, bars, hostals, were all owned by foreigners. They explained that the locals don't like to run or own the businesses. Being Margaritans they just like to party all the time and go and work somewhere when they find they need some more money. And that's how it was - great!

The lady of the Posada told me one day she'd found out the price of a vacant lot a couple of blocks away. It was amazingly cheap.
She insisted I should buy it, build a bar or hostal on it and move permanently to Margarita! Well the temptation was huge!

That's exactly what her Canadian husband had done. He'd visited, fell in love with the place, bought Posada Caney and married a local.
He was in the final stages of selling his dentistry practice back in Canada.

I had some serious thought about that one! He'd gone as far as explaining there's hardly any fresh water on Margarita, it mostly comes by pipe from the mainland. And it's not continuous. So any new building has to have a huge water tank of legal size buried underneath before permission is given for building or running the business. That normally costs the same as the purchase price of the plot of land. Which explains the cheap prices.
I did a lot of research!

Specially after a bus ride to Porlamar, the capital. I stayed in Juan Griego about 2 weeks, taking a few bus rides to visit the capital.
On one journey back to Juan Griego the fare collector on the bus refused to take any money from me. I didn't have enough Spanish to work out why. I asked the driver and understood enough to hear him say it's entirely the fare-collector's business.
So I thought - free bus rides as well! Maybe I should stay.

I'll venture another comment.

Will you be travelling alone? (I assume you're male).
Well, if you're on your own in South America, specially with your own transport (I was on public transport), all you have to do is keep to the small towns that don't feature much in the guide books and you'll have plenty of adventures and see lots of places.

The other side of the coin is you need to have your wits about you at times. Because local women as adventurous as you are will see you as a ticket to 'The West' or wherever you come from. I got into one fight, in Peru, because I'd been dancing (unknowingly) with a barman's girlfriend. But I also learned, I think, that South Americans will only get into a fight when they are so blind drunk that they are completely incapable of fighting. So that was alright then.
I felt pretty safe everywhere.
But did witness an unsuccessful handbag snatch in Caracas. So as usual it's the big cities where you need to be careful.

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TTR250 - London to Cape Town