WotC accidentally left the text of the new Dive mechanic in the release marketing materials (though they’ve now pulled the offending PDF). This is the evasion mechanic that’s standing in for Flying due to the general lack of flying creatures underwater:
“When this creature enters, if it devoured a creature, it dives.” (It can’t be blocked until it deals combat damage while attacking.)
Interesting mechanic, we’ll have to wait and see what makes a creature dive. I guess repeatable dive can be powerful, depending on how easy it is to get