[@Halvtand] First things first. He is not a robot. More like he got a permanent metal encasing as a skin, permanent armor. His legs when he transforms are like that to allow him to achieve greater speed and to increase supportibility of his weoght.But inside he's all flesh and blood and mush. So same as the venom concept you mentioned except with a constant metal armor. To say one last thing on this, even if he had 0 metal on him, all his limbs and such would function the same except look more fleshy. So same as the nymph who got metallic claws as weapons he got metalic body armor.

And about sea dweller... The basic concept was that he could swim fast by rotating limbs as propellers. But that seems a lot like transformers now that I think about it.

Now that I explained, if it still bothers you I'll go with the venom idea you proposed. And I also changed the special traits. If you have a problem with one in particular tell me and I'll remove it.