dive training

Tideriders may spend years at sea without seeing dry land. For that reason, it is critical that they learn as early as possible not only how to swim, but also how to fall into the water without getting hurt. Swimming lessons take place in bowls made of old sailcloth, increasing in depth as the child becomes more proficient. The deepest bowls are for preliminary dive training; children are dropped into the water until they master the correct posture, then tossed so that they must adjust their posture in midair. The final stage of training involves ambushing the proto-sailors. Since most falls are accidental, sailors must be able to recover from an uncontrolled tumble into a smooth fall. To that end, youngsters are pushed or thrown from the ship at moments they do not expect, and must perform five satisfactory dives before they are allowed on deck unsupervised.
In theory it is strongly frowned upon to use the "fall check" stage of dive training as a means of petty revenge, but in practice this is hard to regulate. It has been observed that in any tiderider family, the least-liked kids are tossed most often.

...The ship stopped tipping suddenly, and Delu slammed face-first a second time into the spar.  With a thunderous crack the lookout tower tore itself apart an armspan above her head.  The remaining stump recoiled, finally flinging her loose.  She flailed in the air an instant.  Sound stopped suddenly.  The sky dropped out of sight, replaced by churning water.
Too dazed to panic, Delu reacted as she had been trained.  She closed her eyes and tucked herself into a ball, hastening the spin.  When she gauged her head was rising again she straightened out, arms and legs wide.  Briefly she looked, saw the sea give way to sky, slammed her feet together and pointed her toes, pulled her hands in and clamped them over her face.
She sliced into the water perfectly...