Me, I'd go bold. Beach cottage does not have to be "New England".
A quick google image search of Costa Rica shows lush rain forest right up to the beaches, waters in every shade of blue, and brightly colored flora and fauna (loved the golden tree frogs!). Bring this in.
Use an aqua placemat as a runner for the nightstands. If the bulb temperature is right (more white than yellow) you could get a waterfall effect with the crystal bases.
Not all wall art needs to be heavy. A poster, or patterned fabric on a thin rod would do the trick. Pick pillow and throw colors from either to tie them together.
I'd had the brief thought of literally bringing the outdoors in... banana leaves or palm fronds, carefully done, could be great decor without going to far... but I'm not sure how well they hold out.
Flowers, particularly local varieties, would bring in an enormous "pop" to the room. =glances at the images page again= Can one dry Birds of Paradise?
One a side note, for a rustic knitted pillow, try a basic basket weave (alternating squares of stockinet and reverse St) with a garter border. Easy to adapt to size and yarn.