1: is it possible to make caves with water in them? I want to have some boats sailing through a nice cave.

2: The quests can buy some sort of stamps or passports, what are those fore?

I hope someone can give me some answers

In answer to your first question.....its basically No.You cannot create caves with water passing through them.The closest you can get to it, is with swimming pools.It is a nice thought though.
The answer to your second question.....passport machines placed in various locations through the park basically mean a free prize for your guests(providing they get a ticket from all the machines used).The main idea of this, is to get the peeps to explore all the different parts of your park, and doing it this way, adds an incentive.

That is exactley what i would have said but made it shorter Lol.