Make sure you give yourself a good helping of suncream because at such high altitude you'll burn more easily and also the cool air can make you forget you're roasting under the sun. Our entire group had red faces the day after our trek, and a few days later we were all peeling to the extreme. You were warned.
When you arrive in the boat, first thing you have to do is climb some endless stairs with your rucksack. At the middle of the stairs there is a nice fountain, very tempting indeed. It was very useful to wash face and head, but I was warned not to drink from it, as local water might be harmful for foreign stomachs. So bring your own water!
In the Copacabana- Sun Island boat you can stay inside or go up to the deck. If you do so, be careful with the sun (summer) or with the chilly winds (winter), both might be harmful.