Barnakiel's right: mosquitoes are vectors for lots of nasty bugs: chickungunya, malaria, Ross River fever, Dengue, and hemorrhagic dengue.....yellow fever. etc.
However, physical prophylaxis works. That's a combination of screens, bug juice when necessary, and protective clothing
For the boat, how we addressed the situation in the Solomon Is, with their high frequency of malaria, was the following:
We bought a double bed
sized mosquito net (large enough to enclose our cockpit
at the time--you will need more if you've a large cockpit
; the boat at the time was a 36 ft. mono.) The bed
net was soaked in a permethriin plus water
solution, to saturate it. After it was dry, I sewed it to our cockpit awning in such a way as to overlap the dodger
, and put old heavy line in the hem to weight it down so as to seal off the cockpit. It was fun to sit there in the enclosure and watch the flies land on it and fall off, dead.
After wringing out the net into a bucket, I then soaked an ankle length skirt and lightweight long sleeved blouse in the remains. I only wore them when going into the village after about 4 in the afternoon. Didn't wash them the whole time. Washed me a lot!
We also had screens for our hatches, dorade, and even the companionway hatch
I am a mosquito magnet. Jim refers to me as his sacrificial Annode. :-( However, I was not bitten the whole time we were there.
So, with a little creativity, if you take not getting bitten seriously, preventing getting bitten really is doable. It will take some planning and diligence.
Finally, if there are biting sandflies, the permethrins will still work on the net, but I would use cockroach surface spray on the outside of the port screens and hatches. They don't want to fly in past it. I do not know why, but it's worked for us.