To installing a bladder into the existing water tank of a W28 I think you may be able gain enough access to the original tank from under the starboard and aft settee seats without having to compromise or cut any of the visible or structural interior liner.   You should be able to gain access into the top of the water tank from either or both of those locations. 

There should be a piece of 1/2" plywood in the locker bottoms over the top of the tank at these locations, cutaway the plywood using something like a Fein vibrating saw to expose the tank top.  Then cut into the tank.  The current fill is located under the aft part of the settee. 

Kevin 

One of the biggest problems with bladders is that they are very subject to abrasion.  You need to make sure that the space completely around the bladder is very smooth.  If it is going inside an existing tank, then the inside of the tank has to be lined with a smooth plastic type material, such as 1/4" or thicker sheet foam, plastic, or rubber.

Maybe better to cut an opening large enough, and insert a polyethylene holding tank.  There are lots of commercially available sizes and shapes available.