Like alan says, you will not need an extra visa but it wil cost you $25 or equivalent. I suggest you go as part of an excursion as they will take care of all that for you and it will be easier crossing with a party rather than on your own. Crossing the border can take as much as up to 2 hours. the Israeli's are very thorough with their checks ( that is the best way to describe it!) I had a complete nightmare which I can PM you with the details if you decide you do want to make the trip.
Snorkelling can be done form the beach. there are purpose built jetties for you to get in off. This is to prevent peopel tramping over and destroying the coral life. Snorkelling is amazing. The water is shallow but the marine life is excellent.
It will be hot enough in April.