It’s all up for Crabs

For obvious reasons, the Mermaid Leone and her companions are unable to get up onto the ship, so Eolar and Chen lower themselves down onto the water in one of the sea skiffs.
Close up the Mermaids have a distinct greenish hue to their skin. This isn’t unpleasant but both Eolar and Chen are disturbed to see sharp fang-like teeth partially hidden behind their lush lips.
“About a month ago a rift opened up in the sea floor less than a mile away from our settlement.
The water started to be pulled down it and as the new current formed, it started to drag things towards it.
This in itself isn’t so terrible but the detritus has attracted gigantic scavenging crabs. They are mindless but deadly.
Worse, the hole seems to be getting bigger.
It needs to be plugged before it’s irreparable. There are massive boulders nearby but the Merfolk aren’t strong enough to move them.
Worse still, the giant crabs attack and devour any creature that gets too near.
I know it's a lot to ask, but will you help us?”