Helvola is about the smallest hardy waterlily. It is yellow and easily available. I believe you would be fine leaving it in the tub all winter unless it would freeze solid.

Joann Pring is pink and almost as small

There are many waterlilies described as dwarf but sizes do vary. Perry's Baby Red for example would fill up your tub and beyond...

can't help you with lotus, as I am lotus death even with a greenhouse other than to say look for "bowl" lotus. Not dwarf.

Momo Botan is a dwarf lotus and it looks like a giant peony.

I used aquariumplants.com and was very happy with the price, speed of delivery, and health of the 5 small, pygmy water lilies that I got. They have done well so far flowering all summer and this winter putting up a few small pads-we haven't had any really cold weather yet. I also got a little frog with my order that I loved looking at and listening to every night. The only sad part was that he could not call up a lady friend because they don't live here. I hope he will still be here in the spring.