Harry Krause wrote:

If memory serves, my 1998 Merc Opti required a starting battery with
really high CCA, or at least it said so in the manual. I ended up using
a pair of AGM batteries that individually had pretty high CCA's, so I
never had a problem.

But I agree. Deep cycles are the way to go, especially if you have an
engine that starts easily and you don't have to keep cranking to get it
running. I prefer AGM batteries on "reasonably" sized boats.

One benefit of AGM cells is their very low internal resistance which
translates to high peak current capability. External wiring and
connection resistance to the starter can be more a significant limit.

-rick-