Using car battery in boat
 

Not sure of your reasons for not wanting to have a second battery, but
the answer is you can do it if you want to. What I think you would not
want to do is keep a second battery in the boat all year without using
or charging it. If you go this way take the battery out of the boat and
keep it at home where you can monitor it and keep it from going dead flat.

As for swapping the car battery in and out of the boat for multi day
trips (because you don't want to have to store the 2nd battery at the
apartment), try picking up a reasonable solar panel to at least put some
charge back into the battery during the daytime. You should not need to
go too crazy on the system size but don't go so tiny that it would not
do any good. You would just have to accept that you will have to
replace the car battery more often. And don't drop it during the exchange.

Earl