I have really bad reactions to insect bites, especially Mosquitos, which contrary to some beliefs, we do have in the UK. When going on foreign holidays I take antihistamines daily and they minimise the effects. Before I used them my reactions were so bad that I could hardly walk due to swellings caused by being bitten on the ankle. My GP told me that the reason for extreme reactions to insect bites is that some people's bodies produce too many histamines to fight the bite, therefore anti histamines do help. This explains why some people are affected by bites and some aren't. I used lavender oil abroad, as this is supposed to deter some biting insects as they don't like the smell. It works pretty well but the smell does get on your nerves a bit unless you like the scent of lavender.
As ours is a new boat only recently finished we have not spent any substantial time on it so far so it remains to be seen how I will be affected. A friend recently recommended Body Shop Sweet Lemon body cream, which she swears by to deter biting insects, so I have bought some to use on the boat as it smells much nicer than the Lavender oil. I have not tested it yet but will be tripping of soon so we shall see.
I hope this helps and I really empathise with your wife because I have had many a holiday nearly ruined by reactions to bites and the worst ones are around your ankles as they affect your ability to walk without being in extreme throbbing pain.