Given what a lawyer/airboater just mentioned above, it sounds like you will be in the clear. Of course, any lawyer would need to see ALL the facts of the ENTIRE situation to be able to give any truly reliable advice. As would any of us.
I'd not leave any airboat tied up in deep water to a dock WITH THE KEY IN AND TURNED ON unattended for even a couple hours!!!!!!
I'd be VERY reluctant to leave one in the water, unattended, over night with the key off.
As for the bass boater comment... Meaningless in my opinion. There are spiteful and nasty people in every shape, nation, & color on this planet... INCLUDING airboaters. Unless you saw someone throwing a wake in a no wake zone, and got their reg. #, well, that's not even an issue - whoever is crying on that one.
You sold a boat, the buyer left it tied to a dock, and in the morning found it on the bottom?
Sounds to me to be the new boat owner's fault. At least, if I had just bought a boat, that's how I'd see it.... EVEN IF you had told me the automatic bilge pump would make everything alright. I'd not trust the battery, wakes (as you mentioned) or theivery. I'd never have left the boat out.
But that's just me. I'm not an expert.
matt.