Bowser: Giant, spike-studded, horned, hairy, evil, fire-breathing turtle who can come back to life indefinitely, even after falling in lava.
Kappa: Mythical, decidedly small salamander with a bowl of water on its head, which when empty will cause it to die.
I SEE NO SIMILARITIES. THE NAME "KOOPA" IS A PLAY ON "KAPPA," BUT BOWSER IS NOT ONE.