Eventually, BFR will have enough performance margin to return to the launch site on all missions. I suspect SpaceX wouldn't want to invest in any fixed oceanic infrastructure when their next generation system wouldn't have any use for it.
BFR will have enough performance margin to RTL for missions with F9 sized payloads. But eventually customers will want heavier payloads, and that will mean no RTL.
There’s no provision for BFR boosters to land anywhere but the launch site. They’re looking at 150 tons to orbit that way. If a customer wants more, they can put it up on two flights.
Their BFR video showed sea platforms near cities being used for launch/landing for Earth-to-Earth transit, though.
I believe that is more because most cities won't want a really loud rocket launching/landing nearby.