Hello Franck - yes I remember you!
If all you have used is the SDC 2300 the ground may be more mineralized than you think. The SDC ignores nearly all bad ground, one of its best features. If you use a VLF things may change dramatically.
I spent most of my career detecting in Alaska in the rain. Any good metal detector will work in rainy climates with just minimal care. However, being waterproof is a plus in a rain forest, no doubt about it.
Any VLF gold detector listed at http://www.detectorprospector.com/gold-prospecting-guides/steve-guide-gold-nugget-detectors.htm will do the job and people all have their favorites. My favorites change year to year, so it is not the sort of thing there are firm answers about. To my mind nearly all 13 - 15 kHz detectors are close enough I would use any of them and be happy, and the 19 kHz group has a bit of an edge on the tiny stuff. Rather than go on about it all day though I will offer a couple picks.
The Gold Bug Pro (19 kHz) is one of the most popular VLF nugget detectors ever made and used all over the world. A very safe choice. If waterproof was desirable, the Garrett AT Gold (18 kHz) is a very good alternative, one I might use myself in a very wet environment.
Just my opinion as others will have other ideas, no less valid. There are simply too many good detectors these days and that makes it confusing, but at the same time hard to go wrong. The real difference does lie more with the proficiency of the person using the machine than in the tiny differences between the detectors themselves.