Well, the explanation could be a limited number of times that this technique can be used.
But personally I don't need an explanation why they didn't use it before, because it's just story telling, if they had used it before, then this scenario wouldn't be as dramatic as it is now. Everybody would just think “yeah, ok they are diving again, no need to worry”. You know what I mean.
Nope. Nami even said goodbye to Luffy, they were legit scared that they were all going to die and there was nothing they could do… until Jinbe did his thing. Makes no sense to "save this move for later" when you think you are about to die and have time to prepare (they saw the wave approaching, Jinbe had time to make people tie ropes to the main sail etc)