Can you use an external battery charger to "quick charge" the boat battery
(after extended storage) without disconnecting the battery leads, or should
the battery be disconnected first? Didn't know if the charger would have
any adverse impact on the boat's internal charging/electrical system if the
battery is left connected.
Thanks in advance.

Larger boats virtually all have on-board battery chargers, and you do not have
to disconnect the batery cables prior to using a charger.

However, you should not try to charge the battery from the charger and run the
engine (therefore the alternator) simultaneously. One or the other at a time.