Firstly to give you advise of not even thinking of having second line attached.
You should plug in charger every time you take boat out . That if your car is close by.
On my boat I have installed connector that is on outside of boat where I plug cigaret lighter charger.
Charger is basically a voltage booster 15 V ( from 12V to 15 V with current limiter of let say in your case of 7Ah battery to be 1A.
Like that you will never have this issue.
Ideally will be better to have one battery to run motor and other for steering servo and radio.
In this case you will be able to steer boat in right direction but at slowest speed and with some 1min rests so motor battery regains bit of power. That is in case for Lead Acid sealed. On Lipo bateries , when they are flat that is it - not even after rest will regain any power.
Boats that have only one battery , what happens , motor drains battery to the level when radio stops working properly and then radio turns servos to one side , dropping bucket and start running in circles as motor is on with rapidly finishing what is left of battery .
Then you have absolutely no control - that is time to get ready to borrow canoe or swim.