I guess that the Costa vessels must have a relatively shallow draught, i.e. much less is under the sea, certainly much less than above the waterline.
In any case the Titanic looked like a ship (it sank due to carelessness in known ice-infested waters) whereas the Costas look like floating blocks of flats (apartments to our USA members).
Mind you the Costa Concordia sank due to gross incompetence, the only thing that prevented a "titanic" disaster was that it was so close to land that it did not sink far.
Costa Concordia:
Tonnage: 114,147
GT
Length:
Beam: 35.50 m (116 ft 6 in)
Draught: 8.20 m (26 ft 11 in)
Depth: 14.18 m (46 ft 6 in)
Decks: 13
Installed power:
- 6 × Wärtsilä 12V46C
- 76,640 kW (102,780 hp) (combined)
Propulsion:
Speed:
- 19.6 knots (36 km/h; 23 mph) (service)
- 23 knots (43 km/h; 26 mph) (maximum)
Capacity: 3780 passengers
Crew: 1100
RMS Titanic:
Tonnage: 46,328
GRT
Displacement: 52,310 tons
Length: 882 ft 9 in (269.1 m)
Beam: 92 ft 6 in (28.2 m)
Height: 175 ft (53.3 m) (keel to top of funnels)
Draught: 34 ft 7 in (10.5 m)
Depth: 64 ft 6 in (19.7 m)
Decks: 9 (A–G)
Installed power: 24 double-ended and five single-ended
boilers
feeding two
reciprocating steam engines for the wing propellers,
and a low-pressure turbine for the centre propeller;
[3]
output: 46,000
HP
Propulsion: Two three-blade wing propellers and one four-blade centre propeller
Speed:
Cruising: 21
kn (39 km/h; 24 mph).
Max: 24 kn (44 km/h; 28 mph)
Capacity: Passengers: 2,435,
crew: 892.
Total: 3,327 (or 3,547 according to other sources)