Indossa il giubbotto di salvataggio, altrimenti non puoi salpare.

Italian often leaves out subject pronouns because the verb ending already shows who the subject is.

• indossa = (tu) indossa = you wear / put on
• puoi = (tu) puoi = you can

So the sentence is understood as speaking to you without needing to say tu.
You could add tu, but usually only for emphasis or contrast.

Here it is a command: the tu imperative of indossare.

For regular -are verbs, the tu imperative usually ends in -a:

• parlare → parla!
• guardare → guarda!
• indossare → indossa!

So Indossa il giubbotto di salvataggio means Put on the life jacket / Wear the life jacket.

A useful extra point: this form happens to look identical to the lui/lei present form, but here the meaning is clearly imperative because of the context.

Indossare is specifically used for wearing or putting on clothes and accessories.

• indossare un cappotto
• indossare un casco
• indossare il giubbotto di salvataggio

Mettere is much more general and means to put. In everyday speech, Italians do sometimes use mettere for clothing too, but indossare is more precise and often sounds especially natural in instructions or formal contexts.

So:

• Indossa il giubbotto di salvataggio = very natural, precise
• Metti il giubbotto di salvataggio = also understandable, but less specific

Italian uses articles much more often than English.

So where English might say:

• Put on life jacket

Italian normally says:

• Indossa il giubbotto di salvataggio

The definite article il does not necessarily mean a very specific jacket in the English sense. In instructions, Italian often still uses the article where English might not.

Literally, di salvataggio means something like for saving / for rescue.

So:

• giubbotto = jacket / vest
• di salvataggio = for спасе... wait, no — in English, for rescue / life-saving

Together, giubbotto di salvataggio means life jacket.

A very common alternative is:

• giubbotto salvagente

That also means life jacket.

Altrimenti means otherwise or or else.

It introduces the consequence if the first instruction is not followed:

• Indossa il giubbotto di salvataggio, altrimenti non puoi salpare.
• Put on the life jacket, otherwise you can’t set sail.

It is a very common word for linking a warning or condition to a result.

Because after modal verbs like potere, dovere, and volere, Italian normally uses the infinitive directly, with no preposition.

So:

• puoi salpare
• devi partire
• vuoi mangiare

Not:

• puoi a salpare
• devi di partire

So non puoi salpare is the correct structure: you cannot / may not set sail.

It can mean either, depending on context.

Non puoi may express:

• lack of ability = you can’t
• lack of permission = you may not / you’re not allowed to

In this sentence, the most natural sense is probably permission/rule:

• Otherwise, you may not set sail
• or Otherwise, you’re not allowed to set sail

If the speaker wanted a more direct must not, they might use non devi salpare, but non puoi salpare works very naturally for rules and safety restrictions.

Salpare is a nautical verb meaning to set sail or to leave by boat/ship.

It is more specific than a general verb like partire.

• partire = to leave / depart
• salpare = to set sail

So non puoi salpare is more natural than non puoi partire if the context is specifically a boat.

Yes, the verb forms would change.

Formal singular (Lei)

• Indossi il giubbotto di salvataggio, altrimenti non può salpare.

Informal plural (voi)

• Indossate il giubbotto di salvataggio, altrimenti non potete salpare.

So the original sentence is specifically addressed to one person informally: tu.

Yes, giubbotto di salvataggio is perfectly normal.

That said, you may also hear:

• giubbotto salvagente
• sometimes simply salvagente in context

The exact preference can vary, but giubbotto di salvataggio is clear and standard.