Have any story that's water related like? Or you just wanna add that effect for fun? No problem. I will teach you how to do this, like a boss. It's really easy to do, so don't get stressed out.

O1. FILTERS > ANIMATION > WAVE

BEFORE clicking on it, if you only want a SPECIFIC part to be affected,
use the "FREE SELECT TOOL"  and select around the areas you want it to be effected,
THEN go to Filters > Animation > Wave

O2.  CHANGE SETTINGS & PRESS "OK"
A box should appear as soon as you click wave.
Modify your settings and press okay.
 O3. You're pretty much done now. Just save as/export it as a GIF.