We took an Athens-Rome Mediterranean cruise on Marina in 2017. Loved the ship and the experience. We purchased this cruise on board. Nautica is much smaller than Marina but is dated and lacks the feeling of space and luxury of Marina.
Embarkation at Stockholm was quick but there was no welcome cocktail. Our room (B2) on deck 6 was adequate. The bathroom was tiny and had only a shower protected by a cheap shower curtain (no bathtub as on Marina). Storage space was adequate. Our room was cleaned spotlessly every day. One person commented that a verandah is unnecessary on cruises to northern climates and during cooler seasons, and I agree, even though we had spectacular weather (Europe had its warmest weather on record). Our stateroom was near the rear elevator (closer to the restaurants) which was convenient. Nautica has only two specialty restaurants, so we were only allowed two such meals on a 12-day cruise (no surcharge). Oddly, each time we went, there were many empty tables. It should be noted that Nautica and the other three smaller (older) ships are being "reimagined" over the next year or two and the design looks attractive. Try to hold off until the ships are done. The interiors of Nautica are dated and frumpy. For example, on Marina there were real Picasso and Miro prints, and some delightful avant-garde works of art. On Nautica, the art looked like it came from a neighbourhood art show. Yes, this is a "first world" problem, but Oceania are among the most expensive cruises on the market.
PA announcements were appropriate and brief and the cruise director was excellent on them. I was also impressed by the performers. The shows were appropriately scaled and tasteful, like a nightclub rather than Vegas.