I have not had replace my water tanks, but I have done a fair amount of fiberglassing on a wide range of boats, and can tell you that fibreglass is very easy to use and very forgiving. One of the great advantages is of fiberglass is that, although you may cut holes in it, follow a few basic rules and you can repair almost anything back to its original strength. Alternatively - I have seen this when engines are being replaced and squeezed through the space under the companionway - you can repair the cuts by simply bolting a backing plate over the area - not as strong or elegant, but quick.
The catch in your case is that you probably don't want to mess up too much of the gelcoat...there are many ways to approach it...one option would be to glue and encapsulate some plywood underneath (thicken the epoxy with carbosil or similar to ensure you fill all voids), and then cover that with 'glass, to form a "core" to provide great strength and rigidity over the cuts.
You may want to get a copy of "this old boat" (library?), which will give you the basic techniques of fiberglassing; and then get some practice on something!