It says the «waters of the flood came upon the earth» (Genesis 7:10, NAS) and «the fountains of the great deep burst open» (Genesis 7:11, NAS). (redeeminggod.com)
Have you familiar with this imagery behind the waters of the flood in Genesis 6 - 8? (redeeminggod.com)
So you take all this together, and Peter's point is that just the ark delivered Noah and his family through the waters of the flood which threatened to take their life, so also, we too are delivered from the flood of sin that surrounds us, not by water, but by the Spirit of God (1 Peter 3:18). (redeeminggod.com)