I kinda liked
You are the diamond of every day
Fresh water is metaphorically diamond like (exceedingly valuable in an absolute sense, also sparkly, crystal clear)
but in leaky containers you're stored away.
Aquifers leak. As do reservoirs, rivers and bloodstreams and glands in living bodies. Its kinda hard to find fresh water in places that aren't leaky containers.
You slumber 'neath mountains,
Fresh water can be trapped underground, especially under impervious rock.
... float in the sea,
It's lighter than salt water so it floats in salt water, especially in the form of icebergs.
Too much of you kills, too little equally,
Yes. Obvious is obvious ;)
It's not your glimmer, nor smell I desire,
Its the water itself, not its appearance or non-scent, one desires, when you want a drink.
but a day without you is bland entire.
The world becomes "bland" entirely, when too thirsty... and a day without anything to drink is enough to make one very thirsty.