OMC Cobra 3.0L v-belt size

[goboat3l]

Can anyone tell me the size of the alternator v-belt on a 1988 OMC Cobra 3.0L engine? An AC Delco part number would be great, but I'd be happy with any information. The boat store at the lake told me that it is the same as the merc of the same vintage and sold me that belt. That belt is too small. Considering the difficulty of supporting the engine and removing the top of the front engine mount to replace it, I'd like to have the right belt before heading to the lake again.
Thanks.