Any help would be greatly appreciated. I have a 19' 1998 challenger with a merc 150 xr6. about dec. I started having problems with my starting batt. not holding a charge. replaced batt. 4 times since and all the batts. are not holding a charge when the boat is parked on the trailer .I do not have an onboard charger but do top off with regular marine charger every time.when on the water and fishing the batt. charges fine with the big motor I run the live wells and all electronics and it will start fine even if I do not start the big motor for 8 hours. but if the boat is not charged that night the batt. is dead the next morning.if I charge the bat. and do not use the boat for a few days the batt. is dead.

Since you keep going through batteries with the same result, sounds like you have something drawing the batterey down. Do you have a master switch?

Unhook the battery, charge it, and see if it's still keeping a charge the next day. A good multi-meter is a big helper for any boat owner.

You could potentially have some type of small short as well.

Short would be the first thing that came to my mind too. I would check all your cables for nicks or worn areas. Perhaps it is touching something on your boat or something is draining it like SheldonS said. Surely you wouldn't get that many bad batteries in a row, but anything is possible.