Cotton dries faster and keeps you warmer. You'll need a change of clothes, especially socks.
You will only need waterprrofs in rain, any yellow workmans jacket cos if it's raining it's probably cold, if it's windy and raining it's definitely cold, or on a mono.
A woollen 'bbennie' hat that pulls well down. More use than an extra jumper at night.
If it is a racing
mono you'll need waterproof trousers as a minimum, preferabley a decent waterproof jacket as well because it's the new man that sits on the windward side and keeps the boat upright.
If it's not temperate zone add some thin rubber gloves to keep hands warm when wet, and a pair of those yellow rubberised mesh gloves that give your hands far more grip than you can achieve without conditioning of skin and muscles.
in the UK is one Jacket when we cast off, another jacket when we start sailing. That keeps me comfortable in most conditions, even winter in strong winds.
The top jacket tends be a one of those road menders yellow jobs big enough to go over a gardeners jacket beneath it.
You must allow for sailing at dawn on a cool morning, already wet and tired and a little fed up because you haven't got a go at the wheel
yet. Skippers and crew vary enormously. Most don't mind questions but there's a time and a place. When it gets busy all they want to hear is YES SIR.
Once they've got it sorted out then you can gently ask your questions.
Personnaly I wouldn't go on a boat that doesn't have, and has demonstrated, a Radio
and a GPS
system. You should also be shown how to hit the MOB
button before you leave the mooring
and how to call the Coastguard next.
Asoft bag to airline dimensions is the biggest you should take.