HarryMann:
It has been pointed out earlier that engineers using this phrase in this way means 'deceleration' i.e. force (=ma)
Simple way to think about it:
Airplane is travelling down. It makes contact with the water which stops the fall. This causes upward acceleration (possibly to a zero vertical speed).
Strong vertical acceleration means a strong CHANGE in the vertical component of the velocity. It doesn't necessarily mean anything beyond that.