Tom... The main sailing area is about 150 miles north east of the eruption. There are some low lying islands but they are pretty much uninhabited. The island where Sunsail etc are based is Vava'u and that is well sheltered with relatively steep sides to the anchorage. Here's a link to the first day in Tonga from Jean's blog SailingJeannius click newer posts for the rest of our stay.

Couple of google maps screen shots with eruption location below and Jeannius in the distance in the Vava'u anchorage...