They seem to have upped the damage that mirelurks and It depends on how deep south you go of the map as the higher the level the more damage and HP plus I was playing on very hard mode
Their shields can tank more damage and they dish out more damage. They also have more evolutions through levels. Plus, they have boss units now. Milelurk Kings can pop out of nowhere and rip you apart, and Milelurk Queens are more powerful than most Deathclaw types.
Makes sense really, seeing as Boston is a pretty naval-oriented area.