When I enter tribe house [this is from the christmas event] and if there is water in the tribe house, I go in and I sink.I don't have any cheese and I don't press down, I just start sinking.Must be a bug.

Its due to the new physics of tfm.

This started happening on vanilla maps too.Also, once I took the gift, the sham revived few mice and I wasn't able to swim up.I DIDN'T had cheese.And...Noobclicker, can you explain?What exactly happens?

your in public or private when you sank in water for no reason?

Public rooms.Vanilla.

Have you tried switching rooms back and forth or refreshing???

That won't help. Water is water no matter where you go.
The issue is likely a lot of lag. I have noticed if I am lagging, I will sink in water.

Just found one way to cause the bug to happen [you can test it on @3715006]:
Go in the water, open another tab on your Browser, then when you go back to TFM, you'll see that your mouse died [if there isn't any ground under the water] or you'll see it returning from the deep.

Then this isn't a bug. This is the result on the fix of water bug. Water bug was mostly used, by glitching your browser, or switching to another tab, so you flew really high up. Since the fix, now you sink, when you close, or switch your browser.

So this is the famous water bug...I wanted to try it sometimes.