Brian Nystrom,

Yes, I'm starting to think that myself. However, due to the need for a
custom design (the shape I need isn't available off the shelf), my
options are somewhat limited. I could learn to build a "real"
fiberglass foam mold, but from what I hear, it would be a long learning
process, and I'd probably mess it up the first time. The "fallback"
option is to use plywood sheathed in fiberglass, with straight vertical
walls, and add seperate benches.

Meindert Sprang,

Thanks for the advice. I've found some relatively cheap sources of
5-gallon thick epoxy kits on eBay. They're not suitable for wetting
fiberglass, but probably fine for this task.

William Watt,

I haven't actually calculated how much cement I would need to use, but
I have a feeling that it would weight too much. Maybe I should look
into it, because the water will weight around 4000 pounds after all
(though there may be less water if the tub has a curved bottom and
seats).

Let's say the tub is 5' x 6' x 2.5', and for now, it is just
rectangular. The surface area (including bottom and 4 sides) would be
85 square feet. If the cement was 3" thick (I'm not sure if this is
reasonable), the volume of cement needed would be 85 ft^2 * 0.25 ft =
21.25 ft^3. Let's see, 1 foot is about 0.3 meters, so 1 cubic foot is
about 0.027 cubic meters. So 0.57 cubic meters. At 2300kg/m^3, that
would weight about 1367kg, or a little over 3000 pounds.

I think that would be too heavy for my application, though it all rests
on my guessing that the walls would be about 3" thick.

Thanks for everyone for the help.