Does latex perform well underwater?
Latex will survive at first, but you will need to consider the hardness and any other chemicals that might be in the water. Depending on the water’s properties and the grade of your latex, it might be falling apart sooner rather than later.

I would recommend for your application that you consider using casting silicon, which is a two part component that you will mix and pour into forms. Alternatively you can “brush” it onto fiberglas mesh - or even on top of styrodor foam that you have shaped.

There are also silicon paints that are very robust.

EDIT
Latex is an organic material, and organic materials decompose. Depending upon the aggressiveness of the environment this can happen slowly or quickly. Microorganisms can assist this, as can acidic water or water that has chemical additives, such as chlorine.

Regarding toxicity of silicon, some grades, when hardened, can even be used for food preparation and can withstand great temperature changes.