The Bioshock 3 has a lot of difference from the previous games. These changes aren't just in the "new weapons" league, but totally transmogrify (yeah, I look it up from a dictionary :) the game mechanics. Here I want to summarize these differences.
The game takes place around 1973, five years after the fall of Lamb. The city's decaying speeded up due the lack of maintenance and earthquake-like effects of the explosions under the city. The ocean flooded many buildings and almost all of the lower floors. For example, the downstairs of the Fort Frolic Atrium now filled with seawater, constituting a great open water. But the underwater isn't that sanctum which it was in Bioshock 2. Some Splicer grew gills and paddles to live underwater, among the ADAM-mutated sea life--sharks, giant squids, crabs and other sushi material. And player must fight with them.
When I planned the game, I couldn't decide what the protagonist will wear. Ordinary clothes? Then how could he go out in the ocean? A diving suit? Then how could he using crawlspaces or swimming in the water? The I solve the problem: he can use both of them! The player can put on and took off the suit of a Big Daddy anytime when he (Or she, of course. But I hope there isn't any feminist here, because I won't mention it all the time.) is inside the airlocks. Both mode has its own adventages and disadventages.
I already mentioned the underwater combat. This is a new challenge: you have a limited range of weapons, and differently working plasmids. And no Vita-Chambers...
The player can use his Bathysphere to transportation. This isn't that passive, teleportation-like process in the Bioshock 1, but controlling a vehicle capable to explore underwater areas safely or reaching distant locations.
To be continued