I can see where you're coming from, but I can see it being justified as such: Squirtle is an amphibious creature (well, reptilian, but it lives in both water and on land, so...yeah.), and, like real-life turtles, needs to return to the surface to breathe every now and again. Starfy, being an almost completely aquatic creature (due to his gameplay mostly taking place underwater), would be less likely to drown.
Maybe instead of drowning, Starfy can have sinking physics (not sure if that's too hard to implement), as it would better reflect his games. If you stand still too long underwater in Starfy games, you slowly get pulled toward the bottom of the screen. It would take the same amount of time for him to sink as it would Squirtle to drown.
Just an idea.