We're talking about a handgun not a artillery peice, "deployment" is taking it out of a biometric gun safe, 5 seconds max. Also you wouldn't open fire on someone just beacuse they are near your boat.
Where do you aim the gun? At them? At the deck?
They see you are holding a gun, get a different look on their face, and one starts to reach under the seat.
What do you do?
Remember you are in a foreign country and don't speak the language, don't know the customs, cannot read the body language, ...
Nothing. You don't aim a firearm at anything you don't want to destroy. Until they take action like attempting to board the vessel, you don't even want them to know you have it. Keep it concealed until you need it.
In this situation the first thing you want to reach for is the radio. The gun is more for situations where you would be below deck sleeping and hear someone kicking in the door. Going up against a fully armed team of men isn't possible as a single person with small arms, small arms are useful for one or two burglars breaking into boats on the slip though.