If a pulse type charger will not bring a marine battery to full charge,
find the bad cell(s) with one of these:

https://www.amazon.com/East-Penn-Manu-Ba...9H2RG54DRMZYEJW

Then use an ounce of this:

https://www.batterystuff.com/battery-restoration/fluid/BE12oz.html

Just in the one bad cell.

Hook the battery back up to the pulse charger for 2-5 days.

This is kinda “last ditch” first aid for a wet 12v battery but more than half the time will extend the working life.