99% of chagrin sources are voltage controlled if controlled at all. If an
alternator sees a higher voltage then it is set at it simply stops charging,
the same with sophisticated shore power chargers.

The most popular problem with continues shore charging is that a battery will
get a surface charge. This has caused several people to think that their
batteries were good and found out that after a brief time with out a charge
source they could not start.

Most people who leave a boat in a slip will want to keep the batteries fully
charged.
A decent charer will bring the batteries to 13.2 or 13.3 and then supply only
enough
juice to sustain them at this level. If a through-hull should fail, a bilge
alarm, an adequate pump, and a fully charged battery might slow the net
flooding down enough and attract sufficient attention to save the boat.
Starting with a partially discharged battery is an unneeded handicap. If the
battery is discharging to run a bilge pump, but is being simultaneously
recharged, the number of hours the pump can run will be significantly extended.

Starting the boat with the battery charger running can disguise a weak battery.

Turning on the battery charger once the engine is running will not, ordinarily,
increase the rate at which the battery is being charged. See note below about
proper wiring.

There is an upper voltage limit on some of the regulators and controllers used
to regulate charging from alternators, AC converters, or any other sources like
wind or solar. In some 12-volt system cases, this is 26-28 volts. An alternator
pumping out 14-15 volts and an AC converter doing the same could blow out some
of these components. I have direct knowledge of
an alternator that required a rebuild. According to the rebuild shop, the
damage was "consistent with the things we see when people run their battery
charger and their alternator at the same time."

If a boat is properly wired, there may be only minimal risk of screwing
something up by running the alternator and the AC converter at the same time. A
good portion of the new boats coming off the assembly lines and a likely
majority of boats that have had any electrical cobbling done after manufacture
are not properly wired.

When I worked as a broker, I would never start somebody else's boat with the
shorepower running. Why risk it? If a boat wasn't going to start without the
charger running, the time to address and correct that was *before* a
prospective buyer came aboard.