Security guard is not always a principal part of the crew, so if you want to hire him also, you need to make a special request.
It's pretty clear what his job is - to attend to the security of the boat, guests and the crew.
That is especially important when you are anchoring in marinas where there are lots of people and it's possible that someone tries to enter the yacht (mostly because of curiosity and without any bad intentions).
Even though these cases are very rare, some guests don’t feel comfortable and secure without a security guard.
And although such fear is irrational for most of them, for celebrities and very rich individuals, it is good to have a security around to take care of their families and property.
The chance of someone trespassing on your yacht is very small, because there is always someone of the crew on board, but if you can't relax and want to feel totally safe, we recommend you to hire a security guard.