Some things you need to consider:
Your tank's citizens will be very dependent on your tank staying chilled. If you have a chiller failure you will kill all your animals. It will take only a few hours.
As the temp goes up the dissolved oxygen will plummet! The animals will suffocate...
What I'm saying is you need to make sure that your chiller system is entirely wrung out before you start keeping animals.
Wrapping the chiller lines with plastic is a bad idea. Nothing like plastic bags will keep the water from getting to the evaporator coils. Once the saltwater connects with those coils corrosion will promptly start. It will poison the tank water quickly.
There two standard ways to chill saltwater.
1) You use a standard aquarium chiller. They wrap the evaporator,(the cold side), around a piece of titanium tubing. Titanium is impervious to saltwater. The tank's water is circulated thru the tubing where the heat is transferred to the cold refrigerant. If the water is passed thru the titanium tube too slowly it will freeze up. The system will short cycle and may be damaged.
2) The second method is to use tubing rated for the refrigerant. The tubing is then submerged into the tank you want cooled. This is the most efficient but it requires refrigerant service equipment to apply.
In your case I'd suggest you put your evap coil into a small insulated tank, one you could make out of acrylic and insulate. Run tubing out of this little tank and into your fish tank. Use a little circulating pump, it can be a powerhead even. Fill this tank and the hose with propylene glycol. This is an edible liquid with a very low freezing point. It won't freeze around your evaporator tubing. This setup will isolate your evaporator coil from the saltwater without plastic bags.
Whatever you do make sure you run your system for several weeks to make sure all the problems are worked out.
As we are from the same area, I'll offer a warning. I have seen LARGE tickets given to people harvesting animals, water, and fish from tidepools on the coast. Once down by Stinson beach, a fellow showed me a $1000 ticket for harvesting starfish from rocks. The ticket did not have an opportunity to fight the claim, it was from the DFW. You have to have a license to harvest , plus I would specifically avoid the state or national parks. -I would feel gulity for not telling you in advance, especially if you got fined!
I'm not going to be build the tank out of Acrylic anymore because they started doing much smaller scraps than I need. Full sized sheets of Acrylic are around $800. So, I'm going to build it out of plywood. Here are a few 3D pictures I made with google sketch up. I inside of the wood will be epoxied. Bare with me, this is my first time using it. The tank is going to be 43 inches long, 17 inches wide, and 18 inches high. Its going to have an overflow on the back.
Here is a picture of the front/topish.