The site of the impact, Chicxulub crater; is a circle 170km across, with half on the Yucatan Peninsula, and the other half in the water of the Caribbean Sea.
On land, a trough along the outer edge of the crater contains a vast semicircle of "cenotes", deep limestone sinkholes filled with fresh water.
This BBC clip explains:
Many of the cenotes are connected by an even deeper network of flooded caves which leads to the sea. Freshwater percolating down from rain on the surface and seawater flowing in from the Caribbean form "haloclines", distinct layers of water which don't mix. This clip shows the strange optical illusions caused at the boundary between the layers: