Nick's shark 'Dennis the Menace' was featured in an article in MMI magazine some years ago. It didn't have any control over pitch, the thrusters were used to steer the thing via the gills. There was a compressed air ballast system on-board to control depth. It went very slowly anyway, so presumably static forces were adequate to keep the boat level when underway.
With the Nautilus, the problem is intensified with speed. It's the upsweep of the forward rakers that causes the down force on the hull.
How about making a hammerhead shark. If the nose is rubber, you could twist the ends like forward hydrovanes.