Learn Russian: У вас есть список пассажиров?

This is the genitive plural form of пассажир (passenger). Because пассажир is a masculine noun ending in a hard consonant, it takes the -ов ending to make the genitive plural. The genitive case is used here to link two nouns, translating to "a list of passengers".

Why is список in the nominative case if it is what we have?

In the Russian possession construction (у вас есть), you are literally saying "At you exists a list". Because the list is doing the "existing", the word список is actually the grammatical subject of the sentence, so it stays in the nominative case.

Why do we say У вас instead of У вы?

The preposition у always requires the genitive case. The genitive form of вы (you, formal or plural) is вас.

Could I ask this question without the word есть?

Yes, but the meaning shifts slightly. With есть, you are asking a general question about possession: "Do you have a list?" If you drop it and ask У вас список пассажиров?, you are asking about location: "Is the list with you right now?"