Don't beat yourself up about fear. Fear is what has kept our species alive for thousands of years.
However, you may want to speak with someone about the possibility of acrophobia. (I'm not a doctor, but like to play one on weekends.) It doesn't sound like you frightened of the boat as much as the mental image of a bottomless abyss. In clear water, looking down until there's only dark blue remaining can be intimidating for some. As a new diver, stay where you can see the bottom and ensure the bottom is within your skill limit (~ 40' to 60') That way you have a finite bottom and a finite top as reference points. Good luck.
However, you may want to speak with someone about the possibility of acrophobia. (I'm not a doctor, but like to play one on weekends.) It doesn't sound like you frightened of the boat as much as the mental image of a bottomless abyss. In clear water, looking down until there's only dark blue remaining can be intimidating for some. As a new diver, stay where you can see the bottom and ensure the bottom is within your skill limit (~ 40' to 60') That way you have a finite bottom and a finite top as reference points. Good luck.