16 volts is to be expected. 24?? That would indicate bad connections, especially at battery terminals. Continued running like that can destroy the rectifier. Clean them shiny bright and make sure they are tight.
If 16V bothers you, there is a regulated rectifier available, by CDI.
EDIT: I may be mistaken on that. The regulated rectifier may not be available for that engine. If it is, the CDI part number will be 193-1778 (non-regulated one is 153-1778).
In any case, there are thousands of boats running around at 16V-- including mine--with no problem.
