I would agree... check battery voltage and all cable connections... both Pos and Neg.
Inboard, I assume?
Would this be an old School Bendix style starter motor, or a later version of a HTGR/PMGR motor?
Chrysler autos have used gear reduction starter motors for years. In part, this is why a Chrysler product engine would usually fire up with a low battery, whereas the GM products would not.
Some of the early Chrysler Marine I/B's used a Bendix style starter motor.
Today, we'll find the HTGR/PMGR motors taking their place. (much more efficient)
IMO, if we're going to have a slow cranking starter motor (during low voltage), typically it's going to be with the older Bendix style, or one of the non-gear reduction type starter motors.
If voltage is good, and if cables are good, perhaps post a photo of your starter motor.
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