I wouldn't say it was a refusal. It's a pretty complex subject, actually. The convention doesn't cover just shipwrecks of heritage value, but all underwater cultural heritage. For instance, it could include the remnants of an ancient indigenous village.
Canada currently has no legislation that truly protects cultural heritage at the federal level. So that is the first obstacle to ratifying such a convention. It's up to the country to determine for itself whether it has taken sufficient measures to protect all underwater cultural heritage, such as shipwrecks, villages and other remains that could end up in its waters.
I wouldn't say it was a refusal. The current legislative framework is lacking. In fact, another parliamentary committee, the Standing Committee on Environment and Sustainable Development, discussed the legislative shortcomings. This committee recently made some recommendations for Canada to consider implementing such legislation.