Battery capacity

If it’s new, then it could be a bad battery. If it’s an early 2022 model, then it needs copper washers beneath the Mercury terminal block that is affixed to the battery terminal posts. Those copper washers prevent the connection issue and better allow power transmission. Apparently there is a conductive issue between plates and posts that this resolves.  It should have those on 2023 model year boats, unless a technician removed them not knowing why they were there.  We have seen them removed abs the issue appear later also. It does seem to be more prevalent after a few hours use.

Hard to imagine anybody installing a crank battery with only 600 ccas for a high power Mercury engine.  Ideally, you should be exceeding minimum cranking requirement of 800 by at least 150 ccas to allow for electronics, but, hopefully, you will be ok with the 800.