Lefty / Happily Married / Parent of 2 Girls / Christian / Yucata User
Smack dat Ghoulie, Mr. Trolla!
Short answer: No. You can't migrate over water regions. For further discussion on the ruling see here.
As for your other question, the rulebook states that EVERY explorer can both expand AND migrate normally. Their powers are meant to be special options, not limitations to their normal function. Think of it like this: it's a game of checkers, where all the pieces have the same ability...but these "checkers" are printed with unique symbols that allow you the option to do different things with them as well.