archipelago
The Cayos Cochinos are two small islands and several islets off the northern coast of Honduras. They consist primarily of Cochino Grande (the larger, to the northeast) and Cochino Pequeño.
If you are backpacking probably you want to go to the Garifuna island of Chachuate. To get there you need to sort your own transport (local fisherman or try to jump on an existing tour). The closest point for this is Neuva Armenia, however, the boats leave early in the morning (i.e. 8am or so) and the first bus from La Ceiba to Nueva Armenia is not until 9am so you would need to overnight it. Alternatively you can leave from Sambo Creek,. where there are less options but at least there is a bus from La Ceiba at 7am. Alternatively there are La Ceiba based tour operators, obviously these are more expensive.
You should arrange how many islands you'd want to visit with the tour operator.
Scuba diving:
Probably a good idea to bring some cereal bars/chocolate/apples or something to supplement you throughout the days.
If you arranged transport with a fisherman you would have to arrange pick up at the same time eh?