Seasickness is a funny thing (OK, it's not funny, I guess unpredictable is a better word). It can hit you one day and not another, and the size of the boat or water conditions doesn't always matter. Over the years I've lost my lunch on a 20 foot boat on a really rough day, and on a 48 foot fishing boat on a not very bad day when someone puked in front of me. I run a 25 footer and have never had an issue on it, but I can barely stand on the Body Glove's 140 passenger catamaran without getting woozy. I've had customers point at the 40 some odd footer Newton next to us and said they puked up their guts on that boat a year earlier and had no problems on mine, and I suspect some that have lost it on my boat have switched to bigger boats and done just fine as well. I had one captain that worked multiple boats tell me his most frequent cleanups were on the biggest boat he worked, he figured it wasn't the motion of the boat necessarily, more that since there were enough people on board that generally someone would get ill then the sympathetic puking would set in.... so who knows.
Kona has lots of good dive sites anywhere from 45 seconds to 20 minutes or so from the mouth of the harbor, as well as further sites that don't seem to get as much traffic as they did back in the days of cheap fuel. I think all of us do two sites, and at least several of the smaller boats will pull into the harbor for lunch if someone's feeling ill to either let them off for the day or get on solid ground during the lunch break long enough to recover for a quick run to the second dive of the day. The seasick episodes generally seem to occur if the boat is sitting at mooring rather than moving, so I suspect most companies will try to motor around between dives for the surface interval if they're seeing anyone that's green in the gills on board.
In general, the water conditions in Kona are pretty good by Hawaii standards, as well as by standards of lots of places. Most companies will try to dive at sites that are shaded from poor conditions... dive to the north of a point if there's a swell or wind chop coming from the south for example. Every now and then conditions sneak up on you, but it's not a rule of thumb. My observation is that the worst of conditions are generally between January and early March, and that's not all the time, maybe a few to several days in any given month during that time.
If you have issues with seasickness, I'll pass along the recommendation I've heard from the majority of dive Captains I've worked with in the last decade - Meclizine hydrochloride (found in Bonine and less drowsy formula Dramamine), one tablet the night before the dive and another about 2 hours before the dive in the morning before day charters, or one in the morning and another about 2 hours before the charter if you are doing an evening charter. This seems to work well. I've seen maybe all of 4 people lose it after following this schedule in 10 years of doing this, while lots of other remedies seem not to work nearly as well. We'll have people losing it on any number of remedies and will talk them into trying this and they do just fine the next day.
I don't think there's any real hard and fast rules, but if a particular type of boat generally sets me off I'd probably try to avoid that type of boat if I could.
Aloha,
