I have used natural sea water from singapore shores previously for my small 2.5f tank .. Yes they are quite dirty but after running them thru the filter and skimmer the became crystal clear like all saltwater aquarium.
Best water to take from is from the south area sentosa to be exact and during high tide...
Changi beach not so bad ..
Worst is those from north side like woodlands, sembawang , punggol , kranji.
I have kept clowns, tangs and gobys with no problem. GSP , anemony , zoas grows bigger also.
If you have a small tank or for top up ... I guess its ok ..
But for top up we use fresh water ... Not sea water .. Water evaporates not the salt content... Right?