My wife and I have owned hull number 7 of the Ontario Yachts, Ltd Great Lakes 33. Ours is a 1981 with a Volvo TMD40A. We have had her for eleven years. The boats are very high quality, Canadian built vessels. They are balsa cored, not foam.
There is nothing inherently wrong with cored hulls, they just require more attention to prepping thru hulls. The hole for any thru hull needs to be "sealed" with epoxy prior to installation of a transducer or other fitting. As with wood, solid fiberglass or metal any hard impact needs to be evaluated and repairs made before water incursion. With a cored hulls any major impact demands immediate action.
As with any boat, maintenance and an attentive owner are of paramount importance. And a professional marine survey is essential prior to purchase.
I would be happy to answer any question you may have about the Great Lakes 33. PM me if you like.