We have a general rule
of thumb for non sailors or those who are prone to seasickness if they wish to sail with us ,before we leave, whether it be a day sail or a passage
. They are told the length of the trip in miles and time, what the probable conditions will be based upon the forecast
, what they should expect if they get sick, what we can offer them before and during the trip to lessen the impact of the sickness, how effectively the treatment will usually be and that once at sea --we never turn back unless there is a serious medical emergency
which does not include being sea sick. Most decline after a short but thoughtful assessment and a few will go. Everyone is grateful and thanks us for our honesty. There is no reason to be anything other than brutally honest and if the people are your friends, you will certainly keep your friendship intact. Good luck and good sailing.