How to keep a water mob stuck to the seafloor

There needs to be an option to allow a mob to be a water mob, generate in water, but on the seafloor, and have the mob be unable to swim - only "jump". The mob needs to be able to sink to the seafloor when spawned in water with a spawn egg or /summon. Basically, I need a feature to allow having a water mob that is unable to swim (or not float). I'm trying to make a Starfish mob that breathes water, but cannot swim and moves slowly on the seafloor.