Solution: The Nervous Passenger aboard the Ocean Liner

The clues

• 
  GretaThe gardener who talks to her roses.

• 
  AlmaThe retired botanist.

  She was on a bench.

• 
  ElenaThe gallery owner.

  She was on a chair. She was in the Galley with another woman.

• 
  Lena · VictimThe painter who never finishes a canvas.

  The victim. She was alone with the killer. She was beside a deckchair.

• 
  TheoThe paperboy.

  He was in the Dining Saloon.

• 
  QuentinThe butler of forty years.

• 
  MateoThe waiter who hears everything.

  He was in the top row. He was on a chair.

Step by step

The order our solver finds. Each step uses only the clues and what the steps before it proved, and its heading links to the technique.

1. Pinned suspects Mateo can only be in the top row, so nobody else can be there. Placed: Elena: row 3, column 3 (Galley).
2. Shared rows and columns Alma, Lena and Theo can only be in the fifth column from the left, the sixth column from the left and the right column between them, so nobody else can be in those columns. Placed: Mateo: row 1, column 1 (Galley).
3. Room counting The victim, Lena, cannot be in the highlighted cells: her room there could not hold exactly one other person. The cells meant: row 2, column 5. Placed: Lena: row 6, column 6 (Dining Saloon).
4. Pinned suspects Theo can only be in the right column, so nobody else can be there. Placed: Alma: row 4, column 5 (Cargo Hold).
5. Room counting Elena was in the Galley with exactly one other woman. Only Greta can still get there. Placed: Greta: row 2, column 4 (Galley); Theo: row 5, column 7 (Dining Saloon); Quentin: row 7, column 2 (Lounge).