Most older and cheaper glasses are made from Acrylic. These are strong and very clear ‘glass’. They are great for general use but as all things do have some limitations.
Do not put Acrylic glasses in the dishwasher - they will crack and craze. This craze lines will be in the direction that the plastic flowed into the mould when they were made and usually run vertically. Whilst you may not have a dishwasher on your Yacht or Motor boat remember that a hot kettle of water used to wash up is just as bad - if not worse!
UV Light - Most Acrylic glasses do not like strong UV light and may go slightly milky when exposed for long durations.
Acrylic is a relatively cheap plastic and as such is often used by manufactures trying to make a cheap plastic glass. So watch out for this.
Polycarbonate is a more expensive and superior Plastic which is inherently stronger. For this reason Polycarbonate tends to be used for the better quality less price sensitive glasses.
Poly carbonate is used to make Riot shields and Motorcycle crash helmet visors it is thus very strong (literally bullet proof!) And resistant to UV light - it would not be very good if motor cycle helmets clouded over!
Polycarbonate IS very good in the dishwasher in fact I would choose to wash in a dishwasher in preference to hand wash. - Most of our Exceptional Polycarbonate glasses are hand finished with a blow torch to soften and round off the mouth lip and to help remove any seams in the moulding process. It also has the effect of polishing the glass.