Could you put the Lithium Ion batteries in series to get to a desired voltage?
Yes, as long as they are identical and in very close states of charge, you can run any battery in series. When doing so, it becomes even more critical to balance them properly at each charge to keep all cells in all batteries at the same voltage.
I have got the reverse question: would it be possible for Blue Robotics to build the battery as two parts of 133Wh each and connect them when charging and when using inside the ROV.
The main interest would be to carry both halves as carry-on luggage on the plane (2*160Wh max). If there is a team of two then we could even carry two batteries (four halves).
This is something we have considered and begun to explore, but there are some challenges to overcome. Keeping a good balance of price, discharge capability, clean design (that doesn’t look suspicious) and still adheres regulations by the book and in practice is a bit tricky. This also all still needs to fit comfortably inside the existing battery enclosure.