You can either purchase shore excursions from the ship of hire a cab or in the Caribbean, they have vans that will give you a flat rate for sight seeing. In exotic ports where you don't speak the language, it is safer and more informative if you buy excursions from the ship. We really recommend that just two people should occupy a cabin, but on some ships the cabins are more spacious and can handle three people at most unless the other two passengers are children. The mini-suites usually accommodate three people. Don't forget, there's only one bathroom available per cabin.
Every ship has a medical center with at least one doctor and one nurse.
If the ship is very big, there might be two doctors and there will be two nurses. The centers are very up to date but cannot handle major medical emergencies. Sometimes passengers have to be put off ship at the nearest hospital and we've seen passengers medivaced off the ship by helicopter. It is always wise to get travel insurance just for such emergencies. Better to be prepared than sorry.