Cruising World Magazine outline nine simple steps to avoid an infestation. Some great advice here.
Keeping Bugs at BayView Useful Resource
The Boat Galley caution on the use of bug sprays that contain DEET:
DEET and Plastic Boat Equipment
DEET is very effective bug spray, but be very careful where you spray it. DEET will melt plastic glasses and plates, watch crystals, and the plastic cases and screens on many boat electronics. It doesn’t destroy all plastics (it usually comes in a plastic bottle), but always err on the side of caution around anything that is plastic when you’re spraying DEET or when you might have some on your hands.
Even if you don’t use bug spray with DEET, a guest or friend on a trip ashore might. Their overspray could cause damage to your boat and belongings.