A terrestrial being without any biological adaptations for life below water would require a substantial amount of artificial assistance in order to sustain itself.  A vessel such as a submarine provides what is needed to live below water for most terrestrial beings, but this is merely separating the being from the underwater environment.  Regardless of how it would be accomplished a terrestrial being would need all the items it normally requires for terrestrial life, but those items would need to be accessible and functioning while under water.
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