Dan,

I am sure it is plastic although fiberglass came to mind at first.  The two places that water most likely is getting through looks like they just cracked. There is support under it but there were some  soft spots.  I have owned 2 other units with the same style shower and both of them had a soft spot or two.   What is strange the third spot is right on top of a support and it is cracking.  As for underneath I am assuming  it is just supported with 2X4's standing on their sides but don't know that for sure.  The cracks and one hole just busted through so I feel the floor underneath is fine.  You are right plastic won't rot (bad choice of words) but it looks almost brittle.   I can see that happening over a lot of years but not three.  One thing for sure when it is repaired it will have lots of support.  Ya know what is most frustrating is no shower with both of us working presents problems.   Brings a whole new meaning to the word funky!!  At this moment I am going to put some  locktite plastic bonder in hopes of keeping if from leaking.  I don't feel very confident though.  I hope I can find a pan that will fit under the lip of the walls...... and match the sides SOON!!  Where and how is another problem.

Billy

If you are handy at all you could go to an Auto Parts Store and buy a fiberglass repair kit and make a temporary repair for now.It's not that hard to do.

Chances are good that the Loctite is going to crack and come loose from the pan when you step in it. AND, if you go the fiberglas repair route, site prep is critical. I repaired boats for a while, and site prep takes longer than the fiberglas application. Rough sanding or abrasive wheel, get the cracks and holes clean and dry, wipe with acetone or lacquer thinner, wipe with acetone or lacquer thinner again, let that dry, then cut the glas fiber mat to fit, remove mat, lay down polyester resin, put mat back in place, lay down more resin to impregnate the mat, brush smooth or use a plastic spreader. When you mix the resin with the hardener, you gotta get in high gear. You might have as much as 5 minutes working time from mixing. And no, you can't do a bit at a time. The first coat needs to be done at one time. After that, you can go back with another coat of mat and resin as soon as you want to. Sanding between coatings may or may not be required if the first coat is good and smooth. If not, back to the sanding and wiping. No dust is allowed.