Hey shadowfax, i looked into starting a Native saltwater rockpool tank.
(People Correct me where i make errors)
You basically need to keep it as cool as you can. A chiller would be the best (and most expensive) choice. You may get away with having it inside in the winter in a cool room, or you could keep it outside or garage. Some people also have Big bottles of frozen water that they replace each day. Sometimes when my tropical tanks get too hot i put a fan facing into the water which changes the temp quite a bit.
And when your water evaporates, make sure you only replace it with freshwater or else, like coelacanth said, ull end up with the dead sea. You should try keep a big barrel full of saltwater outside with a pump running through it to save you travelling to the beach each time you want to do a water change. But you will probably want to do water changes quite often. and im pretty sure that the Natural Saltwater and sand taken straight from the beach will have little critters or micro-organisms (whatever you wanna call them - dont know proper term) for your fish or what ever you are keeping, to live on.
This link here might give you some help too http://www.fnzas.org.nz/fishroom/keeping-nz-fish-vt20000.html
All IMO so feel free to tell me where im wrong