Actually, in civil matters, especially dealing with the tax people, the burden of proof is on you.... not them.
I'd also point out that they could easily document your presence, given the advent of digital cameras... all they'd have to do is photograph the anchorage once a day for 90 days... They don't have to prove how long you were there each day, just that you were.
In the particular area I'm thinking of there is no way the local LEO's would have time to stop by and take a picture of who's there once a day. Just so I have an idea, does anyone care to share a general idea of how large a fine we're talking about? two figures, three, four...?