Is there any how to make a player floating on a block like water?

To be honest i don't know too much of what you're asking, I recommend adding extra details in your description about your question and what you've tried so far. To me it looks like you're asking for how to make a block float on water with the water which i think there is an algorithm for water that goes something like this

local x, z = 0, 0
RunService.RenderStepped:connect(function()
    y = 1.8 * math.sin((math.pi * 2) / 85 * x * game.Workspace.Terrain.WaterWaveSpeed / 10) * math.sin((math.pi * 2) / 85 * z * game.Workspace.Terrain.WaterWaveSpeed / 10) + 15
    x = x + 0.18
    z = z + 0.18
    Ship.Position = Vector3.new(0, y, 0);
end);

i hope this helped, it might not have because i didn't fully understand what you're asking

You might consider using a Touch event (of the water block(s)) to start monitoring a player's position. While they are in the water, you could have a BodyPosition in their character that only impacts their Y coordinate and targets where you want them to float. When they get out of the water, you'll need to remove the BodyPosition.

If you wanted them to float up/down (as if floating over waves), you could combine this idea with Stadow's script.

You should use a touched event for when the player touches the water therefore calling a part to the position of the players feet make sure to make it a infinite loop until the player gets out of the water