To get your seawater into the galley sink, you need to do some modifications to your current plumbing. The seawater enters through a through-hull fitting and seacock. Rather than drilling a new hole in the hull for your galley sink, why not simply tie into an existing seacock?
There are important points to look for in a seacock to tie into: Current use and location.
Choosing a seacock is an important part of the process and will affect everything else that will follow. The water to the galley sink will be siphoned off from whatever was already using that through-hull. Through-hulls are sized appropriately to the device that they are feeding, so in a sense you will be stealing needed water away from another part of the boat. It is a good idea to pick a through-hull that is used when the galley sink will not be used. A safe choice is the air conditioners through-hull. When you are in a marina, you will be plugged in and running the air conditioner but you will also have access to fresh water and will not be using the saltwater for the galley. When you are sailing, the air conditioner will be off (because you are away from shore power) so the galley sink will have full authority over the water coming in the through hull fitting.
The next criteria location. Boats are typically setup with seacocks on both sides of the hull. It is good practice to set the discharge seacocks on one side of the hull and the intake seacocks on the other side of the hull. You want to make sure that the seacock you choose for the saltwater plumbing is on the opposite side of all your discharges. The worst thing you could have is the galley intake be located aft of the shower, galley, or toilet discharge. Shower discharge would bring in all the body soil and soapy water. Galley discharge would, in effect, recycle the water you are using to wash your dishes. Toilet discharge is absolutely disgusting and should be self explanatory!
When looking for a suitable seacock to tie into, you know you need one that is on the opposite side of all your discharges. The next criteria is how high the seacock is on the bilge. If the through-hull fitting is just below the waterline, it will come out of the water as soon as the yacht heels over. A through-hull that is lower in the bilge and closer to the keel will stay submerged for longer as you heel over and sail, thus allowing you continued use on either tack.
With the seacock selected, it is now time to tee in the waterline. The best way to tee in is to use bronze fittings. I assembled this setup using a bronze tee with bronze barb fittings for the hose I am teeing into, and a ball valve to the saltwater plumbing. I like to orient the lever of the valve so that any accidental hitting will result in the valve closing instead of opening. In our case, open is up and vertical; should anything bump into it, the valve will turn horizontal and close off.
The tee should be connected after the strainer that way any debris is filtered out before it reaches the pump. It is a good idea to double clamp all raw water fittings located below the waterline. If you notice, I double clamped the barbs of the tee, but I only single clamped the hose leading to the pump. This is because the tee is always exposed to the ocean pressure when the seacock is open, regardless of if I am using the seawater plumbing. When not in use, the valve is closed, so if the hose were to slip off the barb fitting, it would not cause a leak.
While sailing, the seacock is closed, so there is no concern of water leaking. The seawater plumbing is only needed for dishes, which occur around meal times. At these times, the seacock is opened and saltwater will reach the tee. After cooking, the dishes are cleaned with seawater, then the seacock is closed until the next meal. While in port, the valve is shut since the water in harbors is rather disgusting but the air conditioner needs flowing water, hence the double clamps on the tee, but not the valve.
From the tee, the saltwater hose leads to a small electric water pump mounted along the way, then to the next valving station. This station is where the magic happens! Sea water enters from the left, hot water enters from the right. In port, the sea water valve is closed (down position) and the hot water valve is open. Out at sea, the hot water valve is closed and the seawater valve is opened. If anything bumps into the valves, it will simply close the valve (instead of accidentally opening them).
At the bottom of the valving is the waterline that feeds all the boats hot water faucets. This means that in port, we have Hot and Cold water in the galley faucet. Out at sea, we have Salt and Fresh water in the galley.