What critter does this?

[AlaskaDiver]

I saw similar mounds in the Bahamas made by a Southern Lugworm. I don't know if Hawaii has a similar creature.

 

[diversteve]

Could be a Mantis Shrimp - like this one:
http://www.oceanfootage.com/video_clips/RF03_176

They're in your area:
http://the.honoluluadvertiser.com/article/2003/Feb/14/ln/ln01a.html

What's cool about them is that if you throw something metal and shiny into their hole, they'll generally chuck it right back out within a minute or so. But don't get too close, they're pretty aggressive, see better than us and:

are sometimes referred to as "thumb splitters" by modern divers — because of the relative ease the creature has in mutilating small appendages — mantis shrimp sport powerful claws, formed like jackknives, that they use to attack and kill prey by spearing, stunning or dismemberment. Some pet mantis shrimp have managed to break through their double-paned aquarium glass with a single strike from this weapon.

 

[LeslieH]

The two likeliest groups of candidates are lugworms (polychaetes, family Arenicolidae) and acorn worms (phylum Hemichordata, class Enteropneusta). Species ingest sand & mud, digest the organic particles & spew forth the clean sediment. Intertidal ones will produce distinctive mounds of coils at low tide (see http://www.wildsingapore.com/chekjawa/text/s311.htm); submerged ones produce sand eruptions as in justleesa's video.

 

[justleesa]

Thanks Leslie!