Chasing M2/P100 ROV Underwater Drone Robot Part - Battery.
The higher capacity is of course the version you would want, if you can afford it, to increase your dive times significantly, according to Urban Drones, it's only $160 more than the standard battery. The difference in weight is 1.4 kilograms to 1.9 kilograms, which only tends to be a 1.1 pound difference, so I wouldn't think that messes too much with balance or propulsion of the Chasing M2.
Even though this is listed up on eBay, be sure to check other retailers, even Chasing on their official website has it for a cheaper price, seems as though the eBay seller is kind of price gouging, the replaceable battery starts at $389 on their website. Curious if there's any battery shortages because of the Coronavirus Pandemic, could be the reason for the price gouging?