The term "Frozen water" is only a conjectural term coined by fans and has not been confirmed by official sources (e.g. in-game, Ninja Kiwi blog).
Frozen water is space on a map that simultaneously allows land-only (such as Dart Monkeys) and water-only towers (such as Monkey Subs) to be placed. It appears in Bloons TD 6, and equivalents also exist in Bloons Adventure Time TD.
Bloons TD 6
In Bloons TD 6, frozen water is solely generated when the radius of a middle-path Ice Monkey with the Arctic Wind upgrade or higher reaches into water. This allows both water-only towers (namely Monkey Sub, Monkey Buccaneer, and Admiral Brickell) and land-only towers to be placed.
A Sun Temple constructed on frozen water that sacrifices the ice monkey freezing their water will be affected by Admiral Brickell's Naval Tactics ability and Carrier Flagship's buffs as if the temple had been placed on water. This property is also inherited by its fifth-tier upgrade, the True Sun God.
Frozen water disappears when the Ice Monkey creating it is sold, sacrificed, or moved elsewhere. Any land-only towers on the formerly frozen water except Sun Temples, True Sun God, and Paragons will automatically be sold.
Bloons Adventure Time TD
The underwater land in Underwater City levels except Crystal Cave and Sinking is equivalent to frozen water. Both land-only towers (like Finn and Jake) and water-only towers (like the Cassies) can be placed on this submerged land without any trinkets. Placing Captain Cassie or Commander Cassie on this land will easily net the player an achievement, "That's not how Boats work!".
Lady Rainicorn is also equivalent to frozen water, as any land or water tower can be placed on her.