I'm a bit curious since Neptune does not have a land surface and it is covered with water, I mean water has oxygen right? What will happen if we put some sea creatures in these watery planets? Can they survive or live? I'm not referring to Neptune alone, I mean all the planets that has some large amount of water in it. Thanks.
According to Wiktorowicz & Ingersoll (2006), Neptune is too hot and too dry to form liquid oceans at the present time. This may seem counterintuitive for a planet with a far higher bulk fraction of water than the Earth which is located so far from the Sun, but the water is going to be mixed up with hydrogen, helium and various other materials in the planet. The water is going to be in a supercritical state, transitioning to even weirder forms in the extreme temperature and pressures of the deep interior.
If Neptune survives the post-main sequence evolution of the Sun, it may be possible that it forms oceans of some kind in the distant future. In any case there is very little free oxygen so life that requires it (e.g. fish) would not be able to survive. The known extrasolar Neptune-like planets are much hotter than Neptune as they are much closer to their stars, so the "too hot" problem would also apply there.
A better bet might be an ice moon such as Europa. In particular, the radiation environment of Europa might have led to oxygen being produced and dissolving in the ocean. The pressure would be extreme, though life has been found at the bottom of the Mariana Trench. The precise conditions in the Europan ocean, and the oceans on other ice moons is not well known, so there may well be additional problems. It is not clear that there would be the right availability of nutrients, for example.
Being an ice giant, there is no water on the surface of Neptune. Having a surface temperature of $-201$ $^oC$ any water on Neptune will be frozen. Extraterrestrial liquid water is believed to be beneath the ice surfaces of the Jovian moons Europa and Ganymede. A sub surface ocean has been confirmed on the Saturnine moon Enceladus.
As to whether fish or any other aquatic creatures could survive on watery celestial bodies would depend on the temperature of the water and the availability of nutrients and energy sources (sunlight, geothermal heat, etc.) capable of supporting them. This information is not available to us yet.