The LED modules are regulated and can take any voltage between 7 and 20 or so. The motors don't care what voltage they get- the higher the voltage, the more power. They still deliver plenty of power at the lower voltage of the LiFePO4 cells.
The schematic for the 2.6 Controller board is on the OpenROV Github site in the openrov-electronics folder, in pdf form. There is no current top-level drawing showing the hookups to the controller board, but it is not hard to figure out when you look at the wiring instructions for the DB-25 connector on Dozuki.
The battery you link will work just fine, but.......
Changes like this always have a ripple effect through the entire design. If you just want to build the ROV and get on with flying it, I would stick with the existing 26650 LiFePO4 batteries. If you like tinkering with your own designs, then it is an interesting exercise to design your own battery pack. A couple of issues if you want to use 38120 cells like the one you linked-
- These cells are BIG, so you'll need to design your own tube for them.- They are not designed to stack in series- they have electrical screw terminals on each end
- If you put all these on one side, you'll need a counterweight on the other side, and then your ROV will be way too negatively-buoyant, so you'll have to add a bunch of flotation somewhere.
- If you use only one battery pack, you should bring all the power into the ROV electronics tube using both sets of power wires- the individual power wires are not really rated to take the full ROV operating current for an extended period of time- the design is meant to share the current between the two battery packs.
This isn't meant to scare you off, just to warn you that swapping battery pack designs is not a trivial exercise. If you build your ROV with these cells, make sure you post some photos on the forum of how it turns out. We always want to see new variations on the design!