If you're planning to come in April, see if you can switch your itinerary around to go diving earlier and do your other touring later in the month. Like November, the April sea conditions and the weather can be a little changeable as the winds shift direction from northeast to southwest, and every once in a while certain dive sites, or even a whole trip, can be canceled because of conditions--not often, but it can happen, and when it does, the canceled trips tend to be late-April ones. What also can happen is that a trip may end up being canceled because of an insufficient number of bookings--in fact, this actually happens more often than weather-related cancellations. The earlier the trip, the smaller the chance of a cancellation.
I personally am planning to go dive up in the Surins and northern Similans in April