On a day trip a visa isn't usually necessary, but this may depend on what passport you have. It should only be necessary if you stay overnight.
The Aegli hydrofoil is also doing this trip, for the same price as the excursion boats (35 euros) but you don't have as much time over there and it tends to be full of shopkeepers from Rhodes Old Town out to buy cheap cushion covers and rugs to sell to the cruise ship punters...