Thoroughly endose what Jonathan says. I've seen many a boat wrecked by chewing it up below the waterline, then the expensive repair fails time after time because of the differing levels of water permeability of the materials used.
Leave her well alone!
I'd recommend you to remove the antifouling at the end of the season, (use Stripit and follow the instrctiions - its brilliant and very cheap,- provided you wear enough protective gear) leave out of the water all winter this will give her a good dry out, give her 2-3 coats of Primocon in the spring, (if this amout of paint can keep wood & steel completely dry wehen immersed for years, it'll keep a GRP hull dry as well. Then 2-3 coats of antifouling paint (this of course will only half work anyway).
Best bet is to have her out of the water every winter, if not then every other winter. This lets them have a good prolonged dry out!
Hunter Boats produced very superior hull mouldings (on a par with the later Jeremy Rogers Contessa boats), I have never come across any of them with a significant hull problem, and, I'd bet no one else has either. You just have to look at how well all the Impalas and Sonatas still look and how sucessful they are at racing 30 years after they were built, poorly built boats just couldn't do that.
You'll really enjoy the Sonata, they are magic boats, probably David Thomas's best design.