OK, this is just what has worked for us. We take it every morning at breakfast. Only one cruise, when we had 24 foot waves, did we take an extra at bedtime. You can take up to 2 a day, I believe. Doesn't really matter when we take it since it doesn't make us drowsy...just nice to have a set time everyday so you remember.
Personally, with the high cost of a cruise, I'd rather be safe than sorry. We take it every day, every cruise. The problem with waiting is...once you're sick, you're sick! It's hard to catch up with it...easier to stop the problem before it starts. And it's safe, no side effects, so no problem.
Way back when we started cruising, my Dr. suggested trying it for two days before vacation...just to see if we had any problems with it. We did, and we didn't have any.
Again, just mho. Ask your Dr. for advice. BTW, go to your pharmacist and ask for it...I think it's meclazine? I can get a bottle of 100 for about 8 bucks...MUCH cheaper than the Bonine brand!