Likewise, I have observed that the major 1st World countries have the best visual water conditions as to clarity. What pollution there is is mostly hidden or not easily seen. Fertilizers, heavy metal chemicals, sewage overflows when a major rain storm hits a major city, these are what you get from the 1st World countries. It's just not normally very visible.
The "last-world" countries are - IMHO - more honest about their pollution. They dump just about everything including styrofoam boxes, cups, etc., bags, crates, pallets, and raw sewage, etc., into their waters which are all very visible. However, they are less likely to dump the "hidden" stuff like agri-chemicals, pesticides, etc. as they cannot afford to buy and use these things.
As far as cruising goes, areas that are open to the currents of the oceans tend to have much clearer and ?maybe? cleaner water around them as the flushing
action carries the trash away to eventually wind
up on the big "gyres" in the Atlantic and Pacific (See map here - interactive map - expeditions
Specifically, in the Atlantic/Bahamas/Caribbean, I have seen the best waters in the Bahamas
- great flushing
action from Atlantic around - Dominica
; very low population probably leads to low amount of dumping: Grenadines; again low population along with great Atlantic flushing. And just about any eastern side of an island for the same "flushing action" reasons.
The worst list is quite long with the top spots going to the D.R./Luperon - land locked harbor with minimal flushing and maximal dumping; Trinidad/Chag Bay - they seriously just don't care; St Martin Lagoon
- minimal flushing reasons; St Vincent - they just don't care.
Under the "just don't care" category I found that the locals are way too busy worrying about where the next meal or work is going to be found to spend much time and certainly no or very little money
on "ecological" niceties.
So for practical purposes, I use a personal rule
of thumb that if I cannot clearly see the bottom in 10 to 15 feet (3 to 5 m) beneath my boat I do not swim or run the R.O. watermaker
. And additionally, you need to be wary of oil
in the water from other vessels' bilge
pumping - that oil
can take out your R.O. machine real quick. There are special filters you can install to lessen the oil problem but they are hard to find and not cheap
I did make a big mistake in choosing an 120VAC powered R.O. machine instead of a 12VDC machine. With a 12VDC machine you can make water while on passages between islands where, most likely, you will find cleaner, clearer water.