I have an warranty registration card I forgot to mail in on my 1969 Hydrodyne I-O tournament skier that I still have and ski behind today. I think it might be the first year Hydrodyne used the Merc-cruiser if I am not right let me know. Richard
Check out this section, the answer may be there. Some are not too clear, but you could PM the person that submitted it or 2mercs for help reading them. jim http://www.hydrodyners.com/ads.html
I am not sure when they started using Mer-Cruisers, but I ampretty sure it was before 1969, probably early 60s.