Dvojica prijatelja čekaju u marini.

Dvojica means two, but it is the special form normally used for two male people.

So with prijatelji meaning friends (male friends, or a masculine-mixed form), Croatian often prefers:

dvojica prijatelja = two friends

By contrast:

• dva is the basic form for many masculine and neuter nouns
• dvije is used with feminine nouns

So dvojica prijatelja sounds more natural and standard here than dva prijatelja if you mean two male friends.

After dvojica, the following noun goes in the genitive plural.

So:

• prijatelj = friend
• prijatelja = of friends / friends after this numeral pattern

That is why Croatian says:

dvojica prijatelja

not

dvojica prijatelji

Here it is plural.

The form prijatelja can look confusing because it is the same in:

• genitive singular
• genitive plural

But in this sentence, because of dvojica, it must be understood as genitive plural: two friends.

Because the subject refers to two people.

Even though prijatelja may not look obviously plural to an English speaker, the whole subject dvojica prijatelja means two friends, so the verb is plural:

• čeka = he/she is waiting
• čekaju = they are waiting

In Croatian, čekati often takes a direct object without a preposition.

For example:

• Čekam autobus. = I’m waiting for the bus.
• Čekaju prijatelja. = They’re waiting for a friend.

In your sentence, that object is simply not stated. So čekaju just means they are waiting, and u marini tells you where.

It can mean either, depending on context.

Croatian present tense often covers both:

• they wait
• they are waiting

In this sentence, the natural English translation is are waiting.

Also, čekati is an imperfective verb, which fits ongoing or repeated actions.

Here u marini is in the locative because it shows location.

• u marini = in/at the marina
• u marinu = into the marina / movement toward it

So:

• čekaju u marini = they are waiting there
• if they were moving into it, you would use the accusative form instead

Because Croatian has no articles.

English uses a and the, but Croatian does not. So u marini can mean:

• in the marina
• in a marina

The exact meaning depends on context.

Yes. Marina in Croatian means a marina, a place for boats and yachts.

It is a feminine noun:

• marina = nominative singular
• marini = locative singular, used here after u

Yes. Croatian word order is more flexible than English word order.

The sentence as given is a neutral, natural order:

Dvojica prijatelja čekaju u marini.

But you could also say:

U marini čekaju dvojica prijatelja.

That shifts the focus a bit more toward where they are waiting. The basic meaning stays the same.