Logically speaking, Fujitora SHOULDN'T be crossing calm belt. Because we already know that Navy Ships have Sea Stone build under their ships! To where Sea Kings WOULDN"T sense someone is there above them.
But then yet again, Momonga and his forces were attacked before by Sea King, and Momonga fucked it up. So, I guess it's all up to the plot for Oda.
I'm just hoping Fuji is going for the shichibukai. Otherwise, it's really "meh" to sail doing nothing hahaha, specially since he's the one who has been after shichibukai abolishment. Now that he got it, he better go after them. And not stay on the sideline and letting fodder marines getting fucked by shichibukai and then preach about justice!