This is the mminisub, used in the "Clone Wars - Episode 5".
I made it in MLCad, because i don´t have the right parts to built it. So I really think that MOC isn´t good enought. If you build one of this, please, let me know...
From Wookieepedia, the Star Wars Wiki.
"An aquatic version of the MVR-3 speeder bike, the mini-sub, adapted by the Xi Char cathedral factories, saw action in underwater battles during the Clone Wars.
The mini-sub had armor protection for the droid pilot, as well as a single, big laser cannon. They were transported to underwater battlefields inside specialized carriers.
Many of these underwater craft were ripped to shreds by the riding beasts of the Mon Calamari Knights.
The Xi Char provided underwater conversion kits for the MVR-3 speeder bike to the Trade Federation for not being able to figure out how to make the Manta droid subfighter transform like the Vulture droid."