In broad terms it's correct that earlier in the season offers clearer water than later in the season, but it's actually somewhat more complicated than that. Visibility is best once the southwest monsoon stops blowing. That is, when the weather patterns have definitively changed, and the wind is blowing from the northeast. This may happen as early as the beginning of November in some years, later in November in most years, and one year I was up there in mid-to-late-December when the monsoon hadn't yet completely shifted and the vis was not what we were hoping for. Once the water is flat and the sun is beating down on it for days on end from a cloudless sky, the plankton begin to reproduce in the heated water near the surface. You don't really notice this until it hits some exponential threshold, and at that critical point,we start watching out for mantas and whale sharks to come in and feed. This generally happens some time in February, but there is variation in that as well.