Indeed the pressure is huge down there, and this may represent a challange in inflating the ballon in the first place. Looking up the numbers, it seems the pres

Indeed the pressure is huge down there, and this may represent a challange in inflating the ballon in the first place. Looking up the numbers, it seems the pressure at the Titanic is about 400bar, while the pressure of a CO2 cartdrige is about 800bar, so that works. About the baloon becoming larger as it surfaces, a pressure release valve that vents the baloon based on the pressure differential with the exterior (water) may do the trick.