All you really need is a Circular Polarizer....
I'd recommend B+W F-Pro Kaesemann MRC (about $80 in the 77mm size you need) or the slimmer framed B+W XS-Pro Kaesemann Nano-MRC (about $100 at B&H Photo).
The F-Pro has 8-layer multi-coating. The XS-Pro has 16-layer that makes it a bit more scratch/dust resistant and a little easier to clean.
I think both of them are now "High Transmissive" C-Pols.... which aren't as strong a gray as standard C-Pol, so they "cost" a little less loss of light (about 1.5 stops max, versus 2 stops).
Most other brands of C-Pol of this quality cost a little or a lot more. Heliopan, which use the same Schott glass and use brass frames like the B+W cost 2X as much.
If you think you'll be shooting at the beach in salty air and surf spray... or out in sand storms... you may want a UV/Protection filter. A B+W "010" UV MRC (8-layer coatings) in 77mm size costs about $40, I think. There's also a slimmer XS-Pro, Nano-MRC (16-layer coatings) version for about $5 more.
If you want to be able to take long exposures for blurred water or other effects.... you would want one or two fairly strong, standard Neutral Density filters... perhaps a 9 or 10-stop... or perhaps a 3-stop and a 6-stop that you can stack if you want stronger 9-stop. B+W offers quality MRC versions of those, too.
Graduated Neutral Density are not necessary shooting digital. It's easier, much more precise and better controlled to simply take two shots (or dual process a single shot)... one for the sky, one for the rest of the scene... and they combine the "correct" portions from each image in pot-processing. That's pretty easy with many image editing programs (layers & masks in Photoshop). Many software also have a digital graduate filter that can be used.
But some people still prefer to use Grad NDs... In which case you'd need two or three rectangular ones in 84mm or 100mm size, and a filter holder to install them on the lens. Lee, Singh-Ray, Tiffen (glass) and others make good ones. Most are uncoated and are rather easily damaged/difficult to clean optical plastic. So get a good storage case of some sort to protect them. It's also difficult to shade them very effectively with a lens hood. Get one stop, two stop and three stop Grad NDs... or at a minimum one and two stop. In some brands, there also are "hard", "soft" and "medium" transition Grad NDs. With a wide angle you want medium or soft... not hard transition that might be too obvious in images. (Round, screw-in Grad NDs are available, but they force you to put the horizon line right across the center of every image you take!)