Hello. I just got back from holiday while on Accutane. Like you I'm on 30 mg. It was 30+ degrees every day and I was out in the sun a lot. I was using a factor 50 and kept a cap on nearly all the time to stop any sun damage and I had no problem what so ever. I even got a tan and that's saying something as I always burn while I'm on holiday and nothing no sun burn anywhere just a golden tan haha. It even sorted my dry lips out for a week (until I got back home that is)

I had only been on Accutane three weeks so Im not sure if this made a difference or not but as long as you use a strong sunscreen, moisturise and keep your hair protected as the combination of sun chlorine and sea water is going to really dry it out, you should be okay. I even had a drink or two.