At this point it’s hard to say how well Battlefield 4 is functioning on PC without being a regular player. They’ve certainly patched it a fair few times, but I think the commitment to hold off on DLC releases until the title was fully functioning has pretty much fallen by the wayside.
Naval Strike is the latest bit of DLC to be cruising near to Port Release; though only ‘Premium’ members will get it on 25 March. It’ll become available to everybody else (at a price of $15.00 USD) two weeks after that date. DICE will be streaming gameplay footage from the add-on at 12 noon Pacific Time (about half an hour from when I’m writing this) over at their Battlefield 4 site.
You’ll apparently see “how elite Battlefield 4 players tackle new naval environments, the new hovercraft vehicle, and the new gadgets in this expansion.” People anticipating this bit of DLC seem most excited about the ‘Carrier Assault’ game mode, a re-jigged version of ‘Titan mode’ from Battlefield 2142. The livestream blurb doesn’t commit to whether footage from that mode will be shown.
However, you’ll probably get a look at some, or all, of the four new maps: Lost Islands (fishing villages and a crashed passenger plane,) Operation Mortar (a piratical-ish island complete with old fort and working cannons,) Wave Breaker (a naval base and rocky islands) and Nansha Strike (the largest ocean stretch in a Battlefield map ever, apparently.)
Plus, of course, the new hovercraft vehicle. There’s more about Naval Strike and what it will contain, over here.