I don't have experience with this but I would use the following guidelines:
1. Can the tub be cleaned easily if you are really grungy and leave a film on it?
2. How compatible is it with bath oils or other additives?
3. How easy is it to fill and empty, and is there a danger of spillage if you have the tub inside?
4. Can you lean back and relax in it without the water spilling out?
5. Can the bottom be damaged easily if you use it outside?
Because of these issues, i'd probably go more for a small metal or wooden tub shaped like an old-fashioned hip bath than anything with ridges, bumps, or soft sides.

I lived w/o running water for a time. I used a large plastic bin for a tub. I never filled it very full, so that I could drag it outside to dump.It sounds like if you have an indoor drain (the shower) you might be able to dump the water there. Anyway the bin was about $15 at bi-mart, easy to clean and I used to store stuff when not in use for bathing.

I love the manufacturer's tips and directions, especially this one:
"2. you can take a bath half an hour after dinner, its not available for bath if you are too full, too hungry or drunk."

are you going to be using this inside or outside? If you are thinking about outside I see these plastic water tubs for animals to drink from also metal ones at a farm store. People like to buy them for little pools since they are so durable, the round ones can be about the size of a hottub and the oval ones I've seen are about the size of a clawfoot tub. My grandpa actually had the oval one in his basement. I am unsure about the cost of these though but they would hold up well.

I don't have experience with the portables but will share this in case it's of use to you. At our cabin we only have showers indoors so use a galvanized oval tub, set outdoors in a protected area, we even put a drain & plug in it. Not sure if that would work for your situation or not but when I'm sitting in it, with a towel rolled up under my neck, I'm really comfy for a good long soak.

Seems like a lot of hassle for a luke-warm soak. Right, because if it isn't insulated--no warmth!? Also consider that you have to carefully dry-it-out after every use and make room to set-it-up and store it. I'd rather rent a hotel room or go to a friend's home for an occasional soak than try and rig something...especially in a rental where at least you already have a servicable option!

I'd think a inflatable tub or something plastic would be hard to sit on and relax. I think it'd feel like an infant pool. Maybe a galvanized tub?

What about a birth tub, like people use for home births? I don't know mug about them, but they are comfortable enough for pregnant women, so they might be worth looking into.

I love baths. I don't know if I could live with only a shower.

Careful about portable bathtubs. Water weights a ton (in some cases, literally). My senior year in college, my roommate had the brilliant idea of using a kiddie pool to set up a "hot tub" in the middle of our living room. All was fine until one physics major commented that there must be at least a ton of water in the pool. (All it takes is a 6 foot diameter pool filled to about a foot deep to weigh about a ton.) So be sure to have the "exit strategy" for all that water worked out.

Outdoor bathtub - try a Rubbermaid stock tank with a hose slipped over your kitchen faucet and run out a window for hot water.

The coolest thing ever. Trust me.

From personal experience I can tell you that the only thing they are good for is a wash, not a soak.... because there is no real rigidity in the sides, you can't lean against there or you deform the whole thing and tip/spill. If you can perfectly place it so it is directly next to a wall, then you are potentially okay, but otherwise no go. I honestly wouldn't recommend any inflatable one.

I had the same problem in the last place we rented. I love baths, and took a dozen in the first week we lived in our current place (soaker tub!)

I set up a large plastic bin inside my shower. When I'm done soaking and watching internet tv that is propped on the toilet, i just slosh out the water (takes some muscle), rinse off with the shower (and rinse the bin a bit), then turn the bin over in the shower propped up to air dry. It works fine. My showerhead allows for a narrow stream, which works best for filling the tub.

Also, I rarely tell people I do this because it is quite ridiculous, but you're in the same situation so I shared...

Family lore has it that my grandparents used one of those (now antique) giant coca cola coolers as a bathtub on the porch on the house they were building in the 1950s.

@rahskir you just made my day!

Anyone remember the M*A*S*H episode where Hawkeye orders a portable tub from Abercrombie & Fitch (when they were outfitters)? One of my faves.

I've also been searching for a portable bathtub which an adult could use, for our RV which has a shower. I've learned a lot in my search, (suggestions in this thread have been helpful), but I haven't found the perfect tub yet. I've even considered doggy bath tubs! Yes, the tubs for feeding farm animals are durable, but that rubber smell is intolerable. Some great tubs shown online are concepts only and cannot be purchased.