Starter Custom Character can't dive / go underwater

So the problem I’m having is that my custom character, which is a quadruped, can’t seem swim down or dive underwater. It just sits on top of the water. I’ve tried everything (Changing the density, setting the mass, changing the size of the model) there’s also nothing wrong with the model or humanoid. When I’ve used other quad models, they seem to swim fine and go underwater. I don’t know what I’m doing wrong or if this is an issue with Roblox. This problem defeats the purpose of my game as the custom character is semi-aquatic. Any thoughts or solutions? Help would be highly appreciated.