The area being described above...
Viz here is not related to "the time of day", instead it is tied to the phase of the tide, this varies daily.
Viz is not an effect of the sandy bottom. Near the Fantasy Island shore dive gazebo, it is dramatically affected by the siltation run off from FIs man-made beach and lagoon and ham handed earth moving projects.
The further you get from fi, closer to CCV, the water becomes markedly clearer.
Best conditions are at Slack Tide and environs just East of the (shallow placed) Wreck of the Prince Albert. This zone is much more easily accessed from CCV, but from FI, if you stay shallow and have a good SAC Rate, it's quite doable but it's a hump through the murk for a bit.
There are several other factors to consider in regards to the FI shore dive- it is 1/5 mile from the dive lockers- most divers do this dive as a drop off from a pram- and then swim back to the Gazebo Exit.
There is a lot of fast boat traffic, some from FI dive boats, yachtie idiots on jet skis and children thereof, some FI employees commuting. (You can set your watch by the moron at 4:30 every afternoon) Not a great place for popping to the surface, but no place is good for demonstrating that particular lack of dive skills. It's a non issue, unless you're fiddling with buoyancy checks at the Gazebo- it lies at the narrow constriction at the North end of the FI channel. Stay very near the gazebo until you get sorted out.
The outstanding FI dive op employees always save the reputation of this alleged resort, which otherwise, is just this side of dreadful, Dive with them and see this unique marine environment- like nothing else.. But stay or eat there? Ill advised.
They run dive op scheduling on a Euro/Asian system.