Yeah, that’s definitely the area that is most likely to leak. I find that a cup or two of soapy water on the floor of the packraft works well there – you can hold the packraft up so the water pools over the floor-tube joint and squeeze the tubes while rotating it slowly… the bulk of the water will stay at the lowest point (obviously), but a soapy film spreads out over the seams, making bubbles if there’s a leak. You can also push “down” on the floor to pull it away from the tubes which will open up any holes that might be weakly held closed by the inflated tubes (does that make sense?).