Cave biomes are prioritized over ocean/river biomes when displayed on treasure/explorer maps. This can cause a weird static effect, or even make bodies of water not appear on the map at all.

Steps to reproduce for treasure map: 

1. Use the seed -4392674995263788285 and run the following command: 

   /execute in minecraft:overworld run tp @p -155 49 -74 

1. Open the chest at the given coordinates, then look at the map. Notice that there is no relief showing where the oceans are.
2. Run the following command: 

   /execute in minecraft:overworld run tp @s 425 -11 121 

1. Notice that there is in fact ocean on the map, but it does not show up in any unexplored areas where there are lush caves.

Steps to reproduce for ocean explorer map: 

1. Use the seed -4392674995263788285 and go to spawn (if not there already).
2. Spawn a villager and place a cartography table.
3. Trade with the villager until it unlocks the ocean explorer map.
4. Buy the map, noticing that the ocean monument is apparently on land. Use the following command to notice that the monument is not actually on land: 

   /execute in minecraft:overworld run tp @s 224 56 -112