Your water tanks store your most valuable possession, water! Without fresh water, you will die in a few days from dehydration. For these reasons, your water tanks should meet minimum criteria to be considered worthy of holding such a resource.
First of all, your water tanks need to be made of a material that will not react with the water. Plastic, fiberglass and stainless steel are preferred because they won't interact with the water inside the tank like an aluminum tank would.
The next most important function of a water tank is that it lets you get the water out of the tank. The first way to get water out of the tank is via the plumbing. This is the standard and easiest way to get water out of the tank, as long as the plumbing is working properly.
If the plumbing were to encounter a problem and you can't get water out of the tank, you need a second way to get the water out of the tank. A large viewing port on the top of the tank will allow you to gain access to your water and either scoop or pump the water out with a bailer or hand pump. Keeping an extra hand pump on board that is used exclusively for the tanks (and not the bilge) is imperative. If the plumbing fails, simply open the top access and drop the pump into the tank.
Having an alternate method to access your water is crucial, ensuring that you will never find yourself caught with a full tank and a thirsty crew. Your plumbing may seem fine at the moment with no possibility of failure; but if a hose gets clogged, your pump dies, or you run out of electricity, you will find yourself thankful that you have a contingency plan to access your fresh water stores.