Question

Which feature distinguishes slime molds from fungi?

Answer 1

Fungi are separated from them based on their plate-like cristae in the mitochondria, the possession of a single, posterior, whiplash flagellum (in some forms), synthesis of lysine by the AAA pathway, the use of glycogen as a storage compound, and the presence of the spitzenkorper in actively growing hyphae.
Water molds (assuming you are referring to the oomycetes) have heterokont flagella--one whiplash and one tinsel tubular cristae in the mitochondria, and the DAA pathway for lysine synthesis.
Slime molds also engulf their food, which fungi do not. They also have 1-4 anteriorly directed flagella