You could use some of the pla's (does not dissolve in water) with added metals. They vary from 1.5-4 x the density of normal pla.

Pla is technically biodegradable. But for that to happen it needs to be in a hot compost heat with the right moisture. temperature and bacterial levels.

I live in england and have many pla items outside - if they were water soluble - trust me - this winter, they'd have dissolved :-)