Ok, so I got offer for Lavka.
They have several bundles, it's possible to change the content, so it's more estimate how much it would be.
The cheapest option is 418 CZK/$20/person for buffet, for me, it seems like enough (cold buffet 95g/person, hot buffet 190g/person, side dish 100g, salad 80g, fruits 75g, dessert 1-2g). For beverage, it starts at 244 CZK/$12/person. Other options are up to discussion.
They can arrange boat for 30000 CZK ($1500).
Jaroslav
flock-planning@lists.fedoraproject.org