Sea Sickness
I have done lots of research on the subject, and have a thread somewhere around here on the topic, but here is a synopsis.
Nausea in general is cause by four different physiologic components that are interconnected and often work in concert: the inner ear (i.e. semicircular canals), the gastrointestinal tract (the GI tract = stomach and small intestine), the brain (i.e. the chemotactic trigger zone), and psychological input.
I will address each independently, but like most medical conditions each components will affect the others. And the effects are additive and will often cause activation of the every part of the nausea cycle.
INNER EAR - our sense of balance is almost entire determined by the semicircular canals in the inner ear. Fluid and small hairs in these canals let us know when our head has changed positions. In a boat while we are bouncing around on waves, the canals tell us we are moving, but relative to boat we don't see movement. This in coordination of what we see, with what our inner is feeling causes the nausea. This is why looking at the horizon is advocated for people feeling sick. You also put people in the front seat who are car sick for the same reason. Incidentally after drinking lots of alcohol the viscosity of the inner ear fluid changes and you can experience the "spins". The most important thing to remember is the inner ear communicates through "cholinergic" receptors.
GI TRACT - anyone with Montezuma's revenge can attest that spoiled or contaminated food can give you and upset stomach and a case of the squirts. But most importantly, all serious nausea, regardless of the cause, ends up making the GI tract work backwards. Bile and other intestinal juices flow back in to stomach which will promptly empty via vomiting. So while some nausea can be directly caused by or accentuated by food (this is why greasy meals should be avoided because they take longer to digest and sit around longer) seasickness just activates the GI tract from the other causes of nausea.
BRAIN - the chemotactic trigger zone is an area of the brain that is directly activated by chemicals in the blood stream. Chemotherapy, general anesthetics and an assortment of medications can directly cause nausea through this part of the brain. Additionally other forms of nausea can be linked to this area as well.
PSYCHOLOGICAL - everyone has heard that "it's all in your head". And in some ways they are correct. I once burned a girlfriend with no history of motion sickness on a offshore fishing trip at night. From that day forward, anytime she saw a boat on TV she would get nauseated. This used to happen to chemotherapy patients. Some would get sick just driving by the hospital weeks after their treatment. But now we have got great anti-nausea agents and this doesn't happen anymore.
The treatments -
Ginger - this homeopathic agent works mainly in the GI tract. The Chinese have used it for centuries to settle upset stomachs, and there is now some scientific rationale. It seems to promote the normal flow of intestinal contents.
Pressure points/wrist bands (i.e. median nerve stimulation) - Again there is some research that shows stimulation of the median nerve promotes normal GI flow.
Dramamine/Bonnie/Phenergan (i.e. antihistamines) - The primary action on these medications blocks Histamine receptors (which has little effect on nausea), but secondarily they can cause sedation and some protection from nausea but blocking "cholinergic receptors" in the inner ear. Histamine receptors are highly variable and so are the individual responses. That is why a medication works for one person but not others. More on this later.
Scopolamine (i.e. the patch or in pill form) - The main action of the drug is a primary blocking the "cholinergic receptors". It works very well at preventing the inner ear from telling the brain that it feels motions. Unfortunately these are not the only cholinergic receptors that are blocked. You will lose your sense of balance, your mouth will be dry, you won't be able to focus on small print, and perhaps most disturbing you may have trouble urinating. Short terms these side effects are hardly noticeable, but after a few day of use you may need to go to the hospital for a catheter. On a side note it does not matter where you put the patch. The activity is based on blood levels of the medication absorbed through the skin, and there is nothing special about the area behind the ear other than you won't rub it off easily by accident.
Serotonin Antagonists (i.e. zofran) - these are the newest prescription medications used for the treatment of nausea. They work by blocking activation of the serotonin receptors in the brain but there is some activity in the GI tract. They work great for the nausea from general anesthesia and chemotherapy, and only moderately for motion sickness.
My plan - basically I take a shot gun approach with the understanding that it much easier to prevent than treat. I cover everything I can with drugs and use positive reinforcement for the psychological (or get somebody hooked up to a good fish early). Everyone new one I take fishing gets at least a Dramamine and scopolamine patch. Light meals with lots of hydration and no beer (at least early on). If they start to feel bad I blast them with a serotonin antagonist. If they still feel bad they get the silver bullet (a phenergan suppository) that will at least put them out of misery and lets them sleep it off. On a side note, I have noticed that if someone can fall asleep on a rocking boat for a good nap, that seems to reset the inner ear and they wake up feeling good.
I have had discussions with navy doctors and people in the coast guard to see what they do for individuals with motion sickness and it is mostly nothing. As long as the individual is not dehydrated with electrolyte abnormalities (which can happen with extreme vomiting) they make them tough it out. In most cases after a few days they feel completely fine. This is little consolidation when you are faced with the possibility of 12 more hours with the horrors of the mal de mar.
So to answer you question yes, having sinus pressure can accentuation the accitivity on the inner ear making motion sickness worse. Your best be would be to try scopolamine.
-CP
sea-sick. 
