Marine Algae and Seaweeds; Plantae plus

As a group the marine algae are essential to the function of the whole undersea ecosystem. Even the larger seaweeds are often overlooked when scuba diving or snorkelling. They are in fact hosts to a bunch of interesting small stuff, including Sea Horses and Nudibranch.
Needless to say, a wide variety of marine algae can be spotted around and about most Koh Phangan dive sites.

Technically coming under the category of Viridiplantae: only the sea grass is a true plant

Not all algae are plants! Many of what we would call seaweeds are actually in the phylum Heterokonta...

Most of them use phycobiliproteins as accessory pigments, which give them their red colour!