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Alejandro, "duplex" refers to a simulatneous two-way conversation, such as the one you have with a good telephone call. Both parties can talk at the same time, and both parties can listen at the same time, because each party has one "channel" open to the other party.
Most radio is "simplex", meaning, when one party talks, the other one cannot be heard. You need to let go of the microphone button and switch back to listening, and take turns with each other.
What you heard about repeaters is probably a misunderstanding. Repeaters normally USE duplex, with incoming calls to the repeater on one frequency, and outbound transmissions on another. But, people who are using the repeater normally are switched (automatically, by their own radios) to talk on the inbound frequency and listen on the outbound. From your point of view, the repeater is going to appear to be using simplex--not duplex--unless there is special provision made for it.
I don't know about Mexico but this would be unheard of for a marine VHF in the US.
I would suggest you contact your autorities down there to find out what modes are normal--and legal--for marine VHF, and then find out what your Uniden supports.
True duplex operation on a single transceiver is not possible, unless the radio has at least the ability to receive on one channel while simultaneously transmitting on a second, which means extra electronics, and extra cost. And at least in the US, I don't think there is any market to support that expense, since we switch from transmit to receive mode instead. (Whether the radio switches channels as it does that, doesn't matter. It is still SIMPLEX as far as the user is concerned. Looks, acts, functions, as simplex.)