Tits A Hoy -

: When she prepares a breath attack, swim behind her head to avoid massive damage.

: Watch for her "This won't be pretty!" line. Swim vertically to avoid the massive lightning strikes. tits a hoy

: Combat in Atlantica relies heavily on magic; faster MP recovery is essential. : When she prepares a breath attack, swim

Enter the Sunken Ship and examine the chest to trigger a battle with a giant shark. tits a hoy

If you are following the specific style of the Wilburgur guide:

Atlantica is a non-linear world where you must master the ability (obtained after defeating the first mini-boss) to swim against strong currents.