Where the tank connects to your plumbing is a topic for discussion as the two choices have alternate attributes. The plumbing can either connect at the top of the tank or at the bottom of the tank.
If the plumbing connects at the top of the tank, a hose or pipe will need to be led down to the bottom of the tank and the tanks will drain via siphon. Water being drawn from the tank will flow up the hose and into the boats plumbing. When multiple tanks are connected to the plumbing in a system, they all depend on having a siphon to drain them properly. If an air lock forms in any of the tanks feed tube, that tanks water line will need to be primed to allow it to feed properly.
This may seem rather inconvenient, but there is a reason to connect the plumbing at the top of the tank. If the plumbing were to slip off of the tanks boss, the water in the tank will not drain out. Instead, nothing will happen and your fresh water will remain inside your tank awaiting your use. You can either reconnect the plumbing to the tank or remove water through the tanks top access. Either way, your water will remain safe and stored until you are ready to get the water out.
The alternate option is to connect the plumbing to the bottom of the tank. This may seem the most logical as water will drain out of the boss and into the plumbing, never suffering form air locks or other related issues. If your pump stops working, water can still be removed by opening a drain lower than the tank and letting gravity do the rest!
While this may seem the most logical, it is also plagued by a serious problem. Should the plumbing slip off the boss, the tank will empty its contents and the fresh water will become lost to the bilge. If you loose all your water while out at sea and have no method to replenish your stores once repairs are completed, you will be in serious trouble!
While it may seem trivial, the security of your water is worth the headache of dealing with air locks and the need to prime water lines associated with a top connecting tank. Be sure that the tank has a large viewing port on the top that will allow you to fit a pump or hose into the top to withdraw its contents should your plumbing system fail.