I went on a different ship but general suggestions:

Bring a good sun hat, the sun is strong.
Ditto on sunscreen.
If you're going to snorkle I'd suggest bringing a lycra dive skin. It adds some warmth, protects from the sun and separates you from that borrowed wetsuit.
Consider a walking stick, some of the trails are pretty rough.
For shoes, a pair of Teva-like sandals are necessary for the wet landings. I brought my hiking boots as well for dry landings and was glad I did because of the sometimes rough trails.
Lots of memory cards! And spare batteries. An inexpensive waterproof camera is nice too for underwater photos of the sealions, penguins, sea turtles and iguanas.
If you can, wait until your stop in the small port town to use the internet. It's much faster and MUCH less expensive.