TC3W's in general are "bad" for cat converters, the issue with Marine oils are that they are designed to biodegrade in water and teh formulation are different from oil to oil so there is no way to tell how those additives effect performance. I think it's good to just not use them. Why bother when you can use a good synthetic non TC3W 2 stroke.

 

This is always a good read.

http://www.animegame.com/cars/Oil%20Tests.pdf