I agree, a private boat tour is not really necessary. For some reason that I don't know, on certain days of the week, you can only gain access to the villa by boat rather than on foot through the woods - but the water taxi can take you there as it can on other days when it is open.
As I understand it, the tours of the villa are only carried out by official members of FAI (the Italian equivalent of the National Trust) who owns the villa. If you are a member of such an organisation, I think you are allowed free entrance to the villa and gardens.
We turned up one busy August day in peak season and only waited for a very short while for the tour, you can then spend as much time as you like in the beautiful gardens - its position is absolutely stunning.
All being well, I hope to be in Lenno two weeks today !