As a very basic answer, a 100 amp/hour battery should provide 100 amps for
an hour or 1 amp for 100 hours, in reality of course it would probably
melt if required to do the former. Then you need to factor in age, type of
use, deep charge trickle or whatever. At best it is a guide only as to the
possible maximum the battery can provide.

As I normally daysail I do not need a battery in my boat. But when I
occasionally cruise for a few days, I would like one, but only for the
lights. I do not have other power consuming devices in my boat.

So now the question is this: could I simply move my car's battery to
the boat? Let's say for a four days cruise during summer time. Will it
still start the car afterwards? I guess there will be the pleasant
side-effect that the car will be less likely to be stolen.