On our last trip to PNG in 2010, we were introduced to "Artemedine".
I've answered this in this thread:
http://wetpixel.com/forums/index.php?showtopic=38538
Basically in combination (and only effective in combo like COARTEM), Artemesinin derived drugs have been effective as a TREATMENT, NOT as a prophylaxis. However, using only Artemesinin has caused the disease to become more resistant. To further this, I believe there was an article in Time about Artemisinin resistance strain in South East Asia, mainly Indochina.
Dont get bitten! Use bug spray....all malaria pills have some sort of side effect from halucinations to nightmares....some make you drowsy and malanol says it is without side effects but hard on the stomach....i was in zimbabwe and you could see the mozzies hovering around your legs without landing because of bug spray...
The Anopheles mosquito is crepuscular and bite at night. So avoid activity outdoors in the night and spray down your room. When you want to take pics of those gorgeous sunsets in R4, then wear long sleeve baggy clothes and use some sort of repellent.
In answering the OP's question, you can look at it statistically. The Anopheles mosquito (carrier of the malaria virus) bites at night. Near dusk, coverup with repellent and if on an outlying island, stay indoors and kill every mosquito in the bedroom. If you don't go onto the mainland or exposed to any local population at night, the exposure risk is lower. On a liveaboard, the exposure risk is much lower than being on an island resort with locals around. Managing your activities to avoid the mosquitoes will definitely help. Of course, the choice is yours whether to take that risk.