The Dutch shipyard Oceanco recently unveiled its 284-foot Colosseum superyacht concept, a trideck ship with widely curved aft decks that the designers feel resemble the grandness and stature of a Roman amphitheater. Built with glass sides, the aft-deck pool gives swimmers ample views of the sea, while daybeds, a bar, and an observation deck surround a second pool on the sundeck.

A world-class wellness facility featuring a gym and sauna offers panoramic, ocean-level views as well. Accommodating up to 14 …