1. Can not be placed on any block that is underwater except dirt. Seems unnecessarily restrictive.
2. Sometimes, when placed underwater passages in close quarters to walls and ceilings, the kelp will pop off the ground randomly and when you place other kelp nearby. this was see in a 2 high 2 wide underwater passage.
3. Requested Feature : Placing Kelp on top of kelp to increase its height. (Think Tall Grass)

Kelp isn't finished yet, so it will be improved when we start working more on ocean sub-biomes.