I wouldn't mind giving you some help with river/lake placement, either. But there's two things one needs to know:

- what's the scale of this map? Is it an Indonesia sized archipelago? A Japan sized one? Or something the size of a couple of caribbean islands?
Even for your own guidance, I recommend figuring this out and then place an outline of that part of Earth over your map, as a layer.

- where is this archipelago placed in the globe? Does it cross the equator? Is it entirely on the temperate region, and if so on the northern hemisphere or the southern hemisphere? Are there are very big landmasses nearby?

This would make river placing both more accurate and much easier. Without this, it's too random to be fun (for me, that is )