Currently there is no way to get how many water blocks a sponge absorbed when placed in water. The new event will also be cancellable.

SpongeAbsorbEvent

Block getBlock()

• the sponge block that caused the absorb event to trigger
  List<Block> getRemovedBlocks()
• a list of all of water blocks the sponge removed

Will also have all necessary cancellable methods because it implements cancellable interface