Port Douglas lies on the western edge of the UNESCO World Heritage Listed Great Barrier Marine Park and a visit to it is one of the most anticipated and expensive activities that a visitor can do.
For 200 days of the year the South-East Trade Winds blow. That coincides with our main tourist season - The southern hemisphere Winter.
These strong to moderate winds produce a pronounced chop in the Great Barrier Reef lagoon which is inside the outer reef. It is inducive to producing sea-sickness in some people.
With the average stay of a week visitors can choose a day where in that week the weather will be most favourable?

Keep an eye on the Synoptic Charts.
Found here: Bureau Of Meteorology

Things to look for:
The High pressure system. This one is very high but it is nearly over New Zealand and about to break away from the Queensland Coast.
Also the isobars are not close together.

How can you avoid getting sea-sick? The SE winds spin off high pressure systems that cross the lower half of the Australian continent from west to east and put a ridge along the Queensland coast. At some point the high pressure system breaks away from Queensland Coast and goes over to New Zealand.
It is in this gap that the calmest weather is found.

Chris Dahlberg visited the GBRMP as a crew member on the Martin Cash and the first Quicksilver many times.
He witnessed many passengers being sea-sick on the rough days and felt for them.
Some do's and dont's.
If you think that you may be seasick take your travel medication at the recommended time before the reef trip. This could be the night before? Be aware of side effects.
When you join the vessel don't have tea or coffee.
Before you come to Port Douglas check the Synoptic maps from the weather bureau to predict the gap in the high pressure systems and then book your reef trip.
Some more hints >