That's a great question. It's been gnawing at me too.
The answer probably most depends upon the type of world we're looking at. Is it a water world with extreme depths? Never touching a hard surface would probably have an effect on the development of life. A water world with shallow seas would most likely development much different life. An ice covered water world would also developed different life.
That being said I get the feeling that the majority of life we ever detect on a water world with a reachable bottom is going to look and act a lot like the mud in your local pond. Single celled organisms with some more complex worms running around. That was life on earth for 2.9 out of the 3.5 billion years life existed. A less hospitable planet could see life stuck at that stage for much longer.