strange... is this a new to you motor or did it run ok and suddenly croaked ?
p.s. post exact model/year #s
not related mind you but all I can ad is that when my Merc Classic 50 had the same issue years ago the power pack was defective. never could figure why but it solved my problem.
has suggested do some testing before spending needless money on parts . did you service the engine in anyway ? how about starting with simple stuff like cleaning up all power connections
to the battery to allow max current to properly spin the starter.
unless your engine is underwater

it is very unlikely grounding is the issue. look for POOR grounding instead.