Morning...1)does the engine turn over when hot with mag's off.
2) Ground....is the bat. ground directly to the engine? not to the boat trying to go thru the engine stand rubber.
3) Do the teeth on the ring gear look ok? cause youve replaced the starter.
Ive got a 10 to 1 hemi head that starts with a small dyno-bat battery, so somethings wrong.
If it turns over hot, -but is hard to start the timing could be to far advanced, or the inpluse couplers might need to be 30-deg, for additional retard. timing should be 25 deg.
I woould try (with mags off) pulling it thru when hot by hand, hopeing you dont have a bearing problem insinde the engine or something serious.