If you think about our Caribbean Tropical Storm Season, it gets pretty apparent.
The Orinoco flows Northward off the coast of Venezuela into the Caribbean basin, hitting Tobago's environs while it is still fairly cold, less saline, and dyed light brown from the tannic acid of the roots of the trees that form the riverbanks.
When tropical storms head West from Africa, along the Equator, they usually "lift" North or NorthWest through the Caribbean, often ending up in a band from Belize, Cozumel, Texas, and up our Gulf Coast.
As these Tropical Storms enter the Caribbean Basin along the Equator, many of them never get "lifted" and continue to plod Westward after making landfall over the Northern shore of South America. This feeds the Orinoco's outflow.
Thus, any of the earlier months of our traditional Caribbean Storm Season would increase the chances of rainfall on the Northern coastline of Venezuela, so any time except that would likely be "higher viz". November thru July, roughly.
If you say the "the ideal (highest viz) time to dive there", some may disagree with that, but, anyway...