That for sure if its the worst thing.
One thing to note that is only a small group 24 battery with 800 CCA or 1000 MCA, most everything will reference CCA. I saw the G24 and had to look it up on the website, as it raised my curiosity on how they do 1000 CCA in a G24 lead acid. That would have been impressive, and some mighty thin plates. If you can jumper the the two starting together that might be the quickest test have the genny running and then start the ending with them together. If it stays running then you know it is sensitive to voltage drop, otherwise you could have a bad battery also. I have seen new ones go bad pretty quickly. In a boat in the heat of the engine room and the bouncing around can break things inside. Also make sure to check the fluid levels that they are topped off with distilled water. If you keep the charger on it can boil them off pretty quick with the heat, as many chargers aren't compensated for the high heat. The voltage has to be lowered during times with high heat. Some of the newer multi bank chargers will have heat probes that connect to the battery to compensate the charge during the hot conditions and as they cool will allow more charge in increasing the float voltage.
Starting a sideline discussion here...
I see that the battery only has a 1 year warranty, so that tells me that Interstate seems to have a pretty high failure rate if they won't stand behind it. That is the reason that I purchase the cheap automotive MAXX batteries at Walmart as they have a non-prorated 3 year plus two more prorated. In the Texas heat I would never get three years out of them. Don't fall for the "marine" labels on the batteries. There are people that say only use a marine battery over a an auto battery, however most compare a auto start battery to a marine deep cycle battery. The deep cycle batteries have much thicker plates to stand the higher depth of discharge without any problems. The starting batteries are thinner plates to supply higher CCA ratings. That is why VP states specifically in their manual not to use deep cycle batteries for starting there engines, and use only starting.
I would really like to see two batteries cut in half to compare two starting only batteries between a marine and automotive in the same group size. For example the Interstate MTP-24 against the 24M-XHD, that would be interesting to say the least. They are 0.2lbs different and 50 CCA different. They still aren't quite apples to apples as the marine is a Group 24MS (marine starting) which is 3/4" higher than the auto version. That would explain the weight and CCA difference. The interesting part is the auto version has a 30 month non-prorated warranty with up to 72 months coverage. They cost within about $10 of each other.
Sorry to get off in a rant, but none the less a bit interesting. Cordale nothing wrong with your selection of battery, just running into left field here.