When it comes to creating floral arrangements that are within the coastal chic theme, grasses are going to be a safe bet. Found on the first dune line of most every coast, beach grass is a universal sign that the water is near. Succulents are also interesting choice and can be found in a faux variety quite readily. The problem for most people however is that silk grasses and succulents (and most every other silk flower or plant) are relatively expensive. Not to mention that typically when you are creating an arrangement of sorts, you will require several of whatever you are arranging, and this adds in cost rather quickly.
A budget friendly solution, and a general rule of thumb, is to look to your local Lowe's, Home Depot or WalMart 
(or any inexpensive nursery type store) for real plants. If you don't have a green thumb...no matter, because the plants you purchase for decorating purposes will ideally cost you so little that if they don't make it, you won't be devastated by the loss, financially.
Since I haven't seen beach grass specifically, for sale at any of my local plant dealerships, I recommend looking for another type of grass that you enjoy. The varieties are plentiful and you will surely fall in love with at least one. I happen to like zebra grass and fountain grass quite a bit. Most varieties are under $5 or less and once you are done having them in your house, you can easily plant them in your yard if you like. Serves a double duty which is always the most frugal option.
Succulents can be even less expensive at around $2 for a small size and come in a variety of really 
exciting shapes and colors from pale greens to dark burgundies with light yellow centers. These little guys are an amazing specimen of drought friendly plantings, but they can be tough to plant in certain zones so you should check this out before get your heart set on planting them in your yard.
The trick to incorporating a real plant into your decor or table-scape is to use a material to cover the plastic pot it comes in, and that is removable for waterings. Some options for this are a nice paper or wrapping paper that you like, cut into a square (the diameter of the square should be approx 3x the height of the pot), bunched up around the pot and then tied with twine (or any other material) to hold in place. The idea is that you can simply untie this or slide it off for your waterings. Once the plant is not longer dripping, simply slide it back on. For a nice coastal theme you could buy a scrap of a nautical stripe fabric and do something along the same lines by cutting it into a square and bunching it up around the bot...then tie it. You could also cut a rectangular strip and wrap it lengthwise around the pot and pin it on the inside (similar to turning fabric inside out to sew up the seam) with a safety pin. If you choose to go with fabric, keep in mind that a ragged edge is really ideal for this beachy look - leave it stringy. You could also run a light and uneven coat of white paint (or any other beachy color you like) over the plastic and it would create that shabby sort of look so appropriate for the beach. If you elect 
to paint, then you may want to consider purchasing a bit of floral moss or peat moss to place inside the pot around your plant to hide the exposed portion of the inside of the pot that you aren't able to paint with the plant planted.
Also consider keeping a contingent of ceramic or fiberglass pots on hand that your plants (while still in their plastic pots) will fit inside of. I happen to have seen a variety of ceramic pots available at my local Dollar Store a few days ago, that would have worked beautifully for this.
Keep in mind that you don't necessarily want to actually plant your decorator plantings in a real pot unless you plan to keep it around for a long time - and you have a green thumb. It's easier to water without making a huge mess by removing the plastic potted plant in sticking it in the sink or to plant it outside when you are finished if you don't have to dump and clean said ceramic pot should the experiment go awry. Have no fear if you don't have a green thumb...it takes about 3-4 months to kill a potted plant (not that I know this from personal experience or anything)!
{Image Sources: www.Lowes.com, www.davesgarden.com, www.lowes.com, www.saslandscape.com}
