The aggregate rating of Costa Diadema is build on scorse given by the following portals:
| Aug / 24 | Sep / 24 | Oct / 24 | ||
| cruisetimetables.com | 3.0 / 122 | 3.2 / 122 | 3.2 / 123 | |
| seascanner.com | 7.6 / 155 | 7.6 / 180 | 7.7 / 185 | |
| cruisemapper.com | 4.0 / 1 | 4.0 / 1 | 4.0 / 1 | |
| cruiseline.com | / | 3.6 / 64 | 3.6 / 64 | |
| cruisedirect.com | 4.2 / 61 | 4.23 / 61 | 4.23 / 61 |
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