Generally speaking, a LiPo is considered fully discharged at about 3.0V/cell. In tests that I have performed, I found that the effective punch of most LiPo's tend to drop at around 3.4V/cell under load. The time it takes to drop from 3.4V to 3.0V is extremely fast.
I have read that 3.0V is really a safety buffer and that the actual danger zone is somewhere around 2.5V which suggests that 10V would be a better value to program than 9V, but keep in mind that you are tempting fate at this setting and really aren't going to gain much extra run time, probably just a few seconds if that.
I have successfully drained and recharged cells that have dropped as low as 1.5V but those packs did not last very long, maybe only 100 cycles over a 9 month period.
What you are asking is very doable, but at the risk of shortening the life of your packs. Here is an article that is well worth a read:
IΒ*would suggest using 12V as your LVC setting, then when the LVC hits, let the boat sit for a few moments at rest to let the cells bump up in voltage, then slowly drive your boat back to shore so you don't hit the LVC again.
Another option is to use a 3rd channel on your radio wired into a relay with a small booster pack to help you get back to shore (effectively giving you a spare tank)