Banded Sea Snake
Also known as: Blue-lipped sea krait
Scientific name: Laticauda laticaudata
Sea Snakes are venomous snakes that inhabit a marine environment, although they evolved from land-based snakes, and some species are still more adapted to land than water. While they might resemble eels, sea snakes are not fish. They don't have gills and must surface occasionally to breath air. Most sea snakes are highly venomous, but their bites are rarely fatal, due to small mouths and short fangs. Still, it's always a good idea to give them a wide berth.
It's unusual to see these snakes, as they're quite rare around Southeast Asia. I've seen one around Tioman as well as the one photographed above around Komodo.
Spotted Garden Eel
Scientific name: Heteroconger hassi
Garden eels can be easy to miss, since they tend to hide in their holes whenever anything comes near. However, if you move slowly and use patience, you can eventually get quite close to them. They inhabit sandy bottoms near coral reefs. All garden eels live in large colonies, where each eel will create their own burrow a few inches away from each other.