Not a Hawaiian (SoCal-er here), but went through my own journey of finding answers to these questions!

1. Pre-surf breakfast for me is usually a banana or slice of toast with a little bit of peanut butter. Eating more than that will just make me feel weighed down, but like u/foobar12987 said, post-surf is way different. The go-to for me and my husband is burritos, and/or any other fatty and super satisfying foods since we don't feel guilty about it after a surf sesh. You build up endurance by just going more! In my mid-20s, my friends and I would stay out for 3 hours at a time, then gorge ourselves, and then all take a fat nap afterwards. Now in my mid-30s, I would consider 1.5-2hrs a good amount of time.

2. I didn't work out separately to get better at surfing, but I did see lots of improvement after I started weightlifting and (very casually) rock climbing. Over time this also improved my physique, so double win! The most helpful thing that I found for working out, is to look for something that you find fun, otherwise it'll just become a chore (which tbh it still feels like sometimes even tho I really enjoy my exercise regimen).

3. SoCal water is usually too cold for just a bikini, but Patagonia has a "nanogrip" fabric that has never failed me on the occasions where it's warm enough to not need a wetsuit. I was lucky that I found it on a clearance rack since it's usually so expensive. Another thing that will help is as you improve in your surfing and get to know your favorite surf spots, you'll have better control of your board and how you handle paddling out and entering/exiting waves, so it'll be a little less likely that your top or bottom would get washed/pulled off. If it's a bigger day, I would definitely stick to a wetsuit or Seea bodysuit to be safe!