Hi,
Usually the Peterhof fountains are turned on not earlier than the beginning of May and the hydrofoil companies start the business at the same time. So there is no chance to get to Peterhof by the hydrofoil in early April.
Another transport option (except for the private car or taxi) is the following:
You take the subway down to ?AVTOVO station (red line), cross the street and take one of many minibuses, for ex. #424, #224. They all have big sign ? which means ?fountains. Minibus will bring you directly to the entrance of the Upper Park in 30 minutes. Dont forget to tell the driver where you want to get off the bus.
Also you can get to Peterhof by the commuter train:
You get to Baltijsky railway station (subway station ?BALTIJSKAYA, red line), buy the ticket to ?NOVYJ PETERHOF (means ?new Peterhof) and get into the commuter train. In 45 minutes you will be in Peterhof. After you get off the train you shall take one of the buses that park on the square in front of the train station. Numbers are 350, 351, 352, 356. Fifth stop is yours Peterhof Parks.
Hope it helps.
Personally I wouldn't go to Peterhof in April - there is nothing to see without green leaves, flowers, fountains...cold, wet and windy. Though the Grand Palace is beautiful at any time of the year.
in any case - have a pleasant trip!