Every liveaboard has a first aid box or box containing the necessary means. They often also have additional medication, but that depends on the country as not everything is easily available everywhere. The guides and crew are trained to provide first aid and can also administer oxygen where necessary. This also means that oxygen must be part of the required first aid equipment and that it is immediately ready for use at all times. If you use medication from the boat, remember that the guides are not doctors. They will advise you to the best of their ability, but be sensible and always read the pamphlet.