Hi there, having read some reviews about how rough the seas can be and how awful the seasickness is, I thought I'd chip in with my experience today, while its still fresh!!!
I am no great sailor, infact I usually feel very queasy at least, and major sickness at worst. Today we were out in a relatively small cat (Reef Rocket) in 25 knot winds and apparently 2.5 metre swells, so NOT ideal conditions ... but it was fine. None of my kids were ill either, despite the youngest being 4 and not able to take any remedies.
My tips for anyone worrying about sea sickness are:
1) sit at the back in the open air if at all poss
2) take ginger tablets (my 7 and 9 year olds could take them too)
3) as other people say, stare at the skyline, the clouds or any other visible and non-moving land masses.
Despite heaps of pretty pessimistic warnings from booking staff, the hotel and even the captain, we all felt absolutely fine - which is quite unusual for us. Guess the ginger works, then!
It certainly is not as bad as the seas you encounter going whale watching off Kaikoura in NZ.
Just hope this helps anyone who is worried they will be sick and it will spoil their visit to the GBR. I certainly was after the doom merchants I met, and also after reading some reviews here. Groundless worries!