I'm prone to motion sickness and will even feel the effects on a ship on smooth seas when you really can't even feel any motion but I've never gotten sea sick on a speedboat. But that can vary for everyone, some can feel it on a speedboat and be fine on a ship. Both times we've been to St. Lucia and been in Spencer's speedboat, the water was on the calmer side. The water wasn't "flat calm", but far from being rough. But it is really hard to know from day to day what the water will be like because the seas can change from day to day and for many reasons. Some of the smoothest seas we've seen was our cruise were two years ago, during the middle of September (at the peak hurricane season). Some of the choppiest seas (and I sure felt it on the ship) were a cruise we did in April. When in St. Lucia on Spencer's tours, you will be on St. Lucia's Caribbean (calmer) side. But that would be a tough call.
Do you get motion sickness or were you just asking if it's possible on a speedboat? The two times we've been on the speedboat on Spencer's tours, we have not seen anyone get motion sickness. As I said, I'm prone to it, but for some reason speedboats don't bother me so I've never felt it myself. I do take Dramamine, non-drowsy (over the counter), for cruise ships' motion but haven't needed it for a speedboat. Are you just asking in general or do you get motion sickness? If so, is it bad? Do motion sickness meds or patches help?
I usually take Bonine for the cruise ship and have never gotten sick. We have done sailboats, also with the bonine, and never got sick. I've been on a fishing charter boat in lake Michigan, and the waters were rough, and I was a little green...even with the bonine. So, I guess I was just looking for opinions on the speedboat. My husband seems to think that the motion would be less on a speedboat, since you go fast enough to go "over" the waves, so I guess that's what we will hope for! I think his tour sounds fabulous, so I really want to do it!