check and clean all connections at battery, starter, and alternator.
Pull alternator and take to local autoparts store and have them bench test it for proper output. Can pull the starter too and have it bench tested.
This should help you narrow down the problem.
Note: marine alternators/starters function the same as their automotive counterparts, but have additional and required spark shielding. DO NOT replace with regular automotive starter or alternator or you risk explosion/ignition of gasoline vapors in the bilge.
Check all hose fittings - raw water to pump, exhaust manifolds, etc.
Check prop shaft/seal. Traditional seals will drip a little water. New style seals are totally leakproof. www.skidim.com
can help with more specific tech advise and parts.