"Low voltage" according to the battery gauge is ~3.4V on the battery itself. That's still above the minimum "safe" voltage for a standard lipo cell.
In the RC world, you typically stop to recharge a lipo battery when it hits 3.3V/cell. There are alarm attachments that plug into the balance port on RC lipo batteries that are made specifically for this.
The built-in battery controller also isn't stupid, it's not gonna let the battery actually die. What you see when the battery "dies" (usually) is an automatic shutdown sequence triggered by the low voltage to avoid actually killing the battery.
It'll be fine.
None of my boats run lipo. I just don't need the power nor do I need the light weight nature of lipo. Usually the opposite, I need more weight to get the boats low enough in the water and that's why some of my boats run old lead acid batteries. Lipo batteries are great for lots of things, and I run them in my truck but I just don't need them in the boats. That said, all my stuff is scale, not race or speed oriented.
One option for avoiding the low voltage cutout is using a non-lipo speed controller and putting a beeping voltage monitor on the balancing leads of a lipo. They are pretty loud so as soon as you hear it you just bring the boat in and stop.
Here is one of those beeping voltage monitors. This was just the first one on amazon. You can find the same thing anywhere under gobs of different brands and they all look about the same. I've seen them at hobby stores and hobby expo booths too.
Something like this?
CAMWAY 5PCS 2in1 1-8s Lipo Battery Voltage Tester,RC Low Voltage Buzzer Alarm,Battery Monitor Checker Tester for 1-8s Lipo/Li-ion/LiMn/Li-Fe https://www.amazon.ca/dp/B07DC3LH9C/ref=cm_sw_r_cp_api_glt_i_04EKBXDEJGBR4ZBY4P2B?_encoding=UTF8&psc=1
I use these for checking voltage:
CAMWAY 5PCS 2in1 1-8s Lipo Battery Voltage Tester,RC Low Voltage Buzzer Alarm,Battery Monitor Checker Tester for 1-8s Lipo/Li-ion/LiMn/Li-Fe https://smile.amazon.com/dp/B07DC3LH9C/ref=cm_sw_r_cp_api_fabt1_tbmRFb7HGSMNS
The uruav works great. The buttons are small, but it remembers the setting between uses. So I set mine to 3.8v and don’t mess with it. I can plug in a battery at night and it is done in the morning. Wish I had more of them, but I guess it’s not too bad to “have” to fly before bedtime! 🤪
CAMWAY 5PCS 2in1 1-8s Lipo... https://www.amazon.com/dp/B07DC3LH9C?ref=ppx_pop_mob_ap_share
Have you tried connect it to a different port/slot on the para board? Have you tried connecting it to a battery checker? Something like this that checks cell voltages. You should definitely get a checker of some kind if you don't have one. These are nice, but a bit pricey...
I had a battery last night whose lead for cell 1 came loose from the plug. You may want to check that the leads are seated well. The wires have crimped ends that are held in by plastic tabs, much like what you see here. Be very careful not to create a short to any other pins when checking them, though.
https://www.amazon.com/dp/B07DC3LH9C/ref=cm_sw_r_other_apa_i_IZqrFbF61E4DT
Leave it attached to the batteries while driving, it will start beeping when it gets low. I put velcro on this one to keep it from flopping around. https://i.imgur.com/4e4nufe.jpg
Remember, the current flow through the system will be the same at each part. If you are pulling 30 amps at the speed control, 2 batteries in series will both be flowing 30 amps. For batteries in series, you can think of it as 1 battery.
I know this post is a month old at this point, but thought I would suggest that you get a voltage monitor for the plane instead of relying on a timer. They are small, plug into the balance connector on your LIPO, and beep incredibly loudly when your battery gets low enough that you should consider landing. They are easily audible while flying and only cost a few dollars. Not sure what country you are in, but I bought these from US Amazon:
Okay, here goes.