Castle Rock / Underwater Island is a boat accessible fresh water dive site, located in Ayer’s Cliff, QC, Canada. The maximum depth is over 150ft/46m.
This is without a doubt the best-known site on the lake. This formation, also known as Castle Rock, is located opposite Blueberry Point, a little closer to the eastern shore of the lake.
More details available at Scubarama’s Massawippi page:
in French,
English (Google Translate).