I wear a 4.3 wettie, booties and mittens with the palms cut out of them. I find the only thing that gets cold are the hands but the mittens really help.

I get cold, so wear a 5/3. Good quality one as the cheapies dont fit as well and are colder. Booties compulsory. Mitts with no palms are good for warmth but restrict the hands a bit. Usually only wear them when it's a southerly and/or snow on the mountain. Most of the time northerlies/nor westers in winter are warm enough without gloves. Gath helmet also keeps the noggin warm. On those days when it's absolutely freezing, I have resorted to wearing washing up gloves and they do work well until you get water inside them. They tear easily and you get a hell of a lot of **** about wearing them (especially the bright pink ones), but if you're enough of a muppet like me and can handle some banter, then it's well worth it to have functional hands.
The other thing that makes it a little easier is to take a 2L water bottle full of hot water with you, wrapped in a towel. At the end of the sail, test it to ensure it's not scalding. If it's the right temp, pour it into your wettie before taking it off. Just like taking a pee but cleaner.

Awesome info, thanks!

The needessentials wetties look great!

I also do a fair bit of SUP when it's not windy, so you have the added hassle of needing to vent your clothing after you've been paddling for a while and get hot. I was thinking about getting something with a vertical front zipper (not the chest entry surf types), like possibly a freediving suit, and perhaps wearing it over a thinner wetsuit like the Oneil 3/2 I currently own.

I just got some 5mm Rip Curl booties, so I'm good from the ankles down!

Add an impact or life vest, extra warmth with the added bonus it's harder to drown And helmet like Marty said.

I've had a great run with a vissla 5/4, best suit I've had for surfing/sailing and didn't break the bank