There are some steps but most of the way down to the lake is down a carpeted underground slope.  It is about 100 metres long maybe (If memory serves).  The boats are rowboats so quite small, but our boatman was very helpful.  It is worth it - I found the trip short but very sweet.  The cave is gorgeous and the coolness is a great break from the heat.  Our boatman was friendly and amazed me by leaping seamlessly between Italian, Greek and English!