For nature & scenery, Saronic Gulf Three-Island Cruise (Hydra, Poros, Aegina) is hit or miss.
For a no-planning taste of the islands in one day it's efficient and sociable — buffet lunch, sun deck, three different harbours. The catch: it's the textbook tour-group experience: barely 60–90 minutes ashore per island, a packed boat, recorded commentary, and not nearly enough time to actually feel any one place.