Flexible Water Tank Installation
I have two Plastimo Bladders for extra water when I plan to be on the hook for more than a few days. They do not need vents. In answer to the original question, no they will not take the pressure. Also, setting them up to backfill by using the inlet valve not the proper way to fill them and one mistake will blow them out. You can get away with this pratice on a hard tank if the fill vent is large enough to spill the overflow the tank won''t crack or blow off a line.
Additionally, hoking up to city water direct is another way to sink your boat. Not reocmmended, but if you must, then do two things, and none of them with the tanks on line. Get a 30psi water pressure regulator and put it in line. Second, go to a Home Depot and buy a water sprinkler timer, the kind you put on a garden hose. Water flow runs the timer. You crank up about 100 gals. on the dial and should you forget to turn the water off and any fitting inside the boat fails, you will only dump 100 gals of water in there, most likely not enough to sink the boat and hopefully the bilge pumps will keep up.