The front vanes are closely coupled to the boats C.G (centre of gravity) and hence tend to push the boat up and down fairly level- excellent for depth control, but not so hot for attitude/angle control.
Stern vanes being well aft of the c.g tend to tilt the boat at an angle, so a leveller can easily sense the change of angle of the boat and react accordingly.
For tyre valves, hot foot it round to your local tyre shop. If you ask nicely, you'll probably get a bagful ,as they just replace them automatically when new tyres are fitted, and the old (usually perfectly fine) ones are just binned.
Be prepared for funny looks from them if they ask what it's for!

One final tip, if you have some rate controls on your transmitter, set them to half for the hydrovane control. Better still would be to dial in some positive exponential throw if you have a computer set. I've piloted one of these things, and they are very twitchy indeed.
If you want to havew some real fun, why not fit two servos to the fornt vanes, and mix them so that you roll the boat as well- a true aquabatic.
Andy