This should be obvious.
Agua Laguna is at the end of the first half of the Grand Line. What does that mean, geography wise? It's on the exact opposite position of the globe from Reverse Mountain and Raftel. Raftel is directly behind Reverse Mountain from where the Grand Line is entered.
Now, Iceburg stated that Agua Laguna was getting larger every year, and was the largest it had been in years. Now, if you have very high tide in one part of the world, you obviously have to have very low tide in the opposite part of the world.
Technically speaking, Water 7 and Raftel are on the exact opposite positions of the globe.
So therefore, the reason no one has gotten to Raftel in 22 years is because the water level had risen over the island. Whitebeard, Shanks, and the other two Yonkou are simply waiting for the tide to recede completely, allowing entry to Raftel.
So the island's sunk. Big deal, right? WRONG. This is the Grand Line. What occupies 90% of the water? That's right, Seakings.
So let's assume that there's a Seaking that can ignore Kairoseki. Perfectly possible. Giant Seaking. When low tide is in effect, the Seaking can't protect Raftel. That's how Roger and his crew were able to approach it.
As they say, Timing is everything.