0.7 oz/yd sil/pu. Which side down for bathtub ground sheet?

I would put the PU side up and the Sil side on the ground. My impression of the fabric is that the PU is what makes the fabric waterfroof and the Sil is more for water repellency. You want the waterproof side on the inside where it won’t get as much abuse. The PU side won’t be as slippery either. The silicon sealer might not stick perfectly if the silicon coating isn’t pure. You can also use liquid seam sealer on the PU side. I used a bottle from an old Eureka tent and it worked very well.

I would say you’ll want the PU side as the side you lay on as it will be way less slick than the Sil side.

Another vote for PU side up:

Silicone impregnates the fibers*** (it becomes one with the fabric instead just coating it)…so the idea I have here is that if you put the sil side down, you’ll have better durability against rubbing/abrasion, which could cause the PU coating to flake off.

***This is the case in silnylon. I expect that this is the case here as well. I’m making assumptions, but I’d imagine that they took sil-impregnated polyester and then put a PU coating to one side for added strength/durability

NOTE: This is 100% speculation but I think the other two reasons that others (Thomas and Hoosier) have given would be enough to put the sil side down regardless.