You are correct - HD stands for heavy duty. For boating in hot climates with no threat of a temp below 40 degrees, use 25 W-40 weight motor oil. Cooler climates, use 10W-30. Check out these threads to learn more: http://www.outboardrepairs.com/oil/#cobraomc_oil
I believe HD stands for "high detergent", and there is a difference with or without the HD additives. Your OMC should do fine with an HD oil, and SAE 30 is a great choice considering that's what the manufacturer says use.
