In short, we have had very little mosquito issues sailing, (but have had some biting flies). Here's my advice:

1. Anchor your boat at least 150m off shore, preferably with the wind NOT blowing straight from the closest shore to you. Mosquitos don't travel too far and don't fly upwind well, so don't let the wind blow them to you. It's tempting to grab a ball close to shore, but if the wind is wrong you're going to get bit.

2. Don't go to the beach at dusk. Bugs really get you from just before to just after dusk, so plan around that.

That's about it, those two tips have kept me pretty much bite free for my trips.

The biting flies, however, are an exception. Unlike mosquitoes, the flies can make it pretty far out into a mooring field. Best advice for them is to keep some flypaper traps below deck (hate to lock a bunch of them below decks overnight...) and if you see more than 1 biting fly when you grab a ball, you might move to another spot because they seem to tell their friends and if you start the night with 2-3 you'll end the night with 10-15.