Sunset cruises typically leave from the Old Port or the new port at Tourlos in the late afternoon, sail past Little Venice and the windmills, and anchor for a swim before the sun goes down. Most include drinks and something to eat; the difference between listings is mostly boat size, group size and how much of the evening is spent under sail versus at anchor.
On land, the sunset bars along Little Venice fill early. A boat gets you the same view without the crowd — and the view of Little Venice itself, which you cannot see from inside it.
In high summer the Meltemi can make the west side of the island choppy in the evening. Catamarans handle it better than small motorboats; if you are prone to seasickness, look for a larger vessel.