The winds are pretty light over there at the moment with the SE trades still forming and the SW seabreeze loosing its strength as we go into winter. BUT as we move into winter (when perth starts getting a few fronts) the SE trades will start to kick in and you will have weeks on end where the wind will not get below 20 knots and the nights and mornings will be more like 30 knots. July and August are almost guaranteed with swell to boot. Monte Bello Island Safaris have a houseboat and charter vessel with the cook being a keen kiteboarder so maybe try them.
DEC however do not give permits for kiteboarding charters as they think we move too fast for the poor turtles[}:)], so go on a fishing charter and take your kite gear

. As far as radiation goes the Islands had less radiation than the city almost 10 years ago when it was last tested, just leave any steel on the Islands well alone as this is all still highly radioactive.(think bbq hotplate that does not need a burner)