Swimming is a feature that is seen in most open world games. However, there are a few games that don't feature swimming at all (yes Metro Exodus, I'm looking at you.)
That being said, how to swim in GTA Vice City isn't the right question to ask. The correct question would be, can you swim in GTA Vice City.
Here's what we know!
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Can you swim in GTA Vice City?
The answer is a strong no. Swimming was only introduced in GTA San Andreas. Before that swimming was lethal to the player.
Although swimming in GTA Vice City isn't an obstacle like it was in the original GTA and GTA II, you'll still sustain damage for standing in water if the level is up to your chest.
Interestingly enough, there are boats in GTA Vice City so you can technically navigate through the water bodies that are present on the island.
Having said that, you can swim in GTA Vice City, provided you install some mods on the game. However, modding the game comes with its own set of risks and issues.
And that's something I know most people would love to avoid. So yeah, you can't swim without mods.
And with the GTA Definitive Edition coming out, we won't be seeing the swimming mechanic in GTA Vice City or GTA III since it will be a direct remaster.