Do these have any kind of liner?

Yes. The liner is soft and elastic, allows for maximum breathability so shorts dont balloon underwater and leave no sand.

What is the inseam length on these?

Approximately 7", depends on different size. If youre looking for a modern fit not looking like a long baggy board shorts, above knee length is the style youre looking for.

Is the waist elastic?

Yes, with adjustable drawstring. The fabric is no stretch. If you prefer 4-Way stretch swim trunks, welcome to maamgic store and we have many stretchy swim trunks to choose.