Fuel tank replacement help

[crsavage1]

So needing to replace the fuel tank in my 1968 19ft and have a few questions. Where the tank was mounted is full of about 3-5 inches of water in a supposedly sealed area. Is this probably from just intrusion through the console or something else? Also it was just on top of the plastic mats in the picture which I assume is not standard?

Any recommendations on where to source a new tank? Also a little concerned about other parts of the floor, is it probably better to replace it before going back with the new tank etc? It appears as though the floor is some sort of sandwiched foam product but not quite divinycell...hard to tell.

Below are the pictures of where the tank was and the tank itself. Any insight and advice would be very helpful.