In episode 3-2 you will continue playing as Jill back on the Queen Zenobia. You will eventually arrive in a large grandiose hall. Go up the stairs, around the walkway, through a door, and continue up more stairs to enter the Solarium. You cannot swim in the pool at this time. Instead you need to go into the small room to the south and activate a switch. At this point, continue with the story.
In episode 4-1, you will find yourself naturally back in the large hall. Go back into the Solarium and you can now swim in it to receive the trophy and gain access to some weapon parts and ammo. (Careful for the 1-shot kill enemies that will descend from the ceiling afterward.)